With a wildlife guide, I stepped on a journey to find one of the rarest creatures in the world ----the spirit bear, a walking contradiction--a white black bear. Neither albino   nor polar bear, the spirit bear is a white variant   of the North American black bear, and it's found almost exclusively here in the Great Bear Rainforest. At 25,000 square miles ---- one and a half times as big as Switzerland--the region is a spooky , wild, mysterious place: there are wolves here that fish. Deer that swim. Western red cedar trees that have stood a thousand years. And a black bear that is white. Scientists know how black bears are born white. They're just not sure why. The phenomenon, known as Kermodism, is caused by a recessive mutation   at the MC1R gene, the same gene associated with red hair and fair skin in humans. To be born white, a bear must inherit the mutation from both parents. The parents themselves don't have to be white. They just need to carry the recessive mutation. So it's not uncommon for white bears to be born to black parents. White fur happens in only one of every 40 to 100 black bears on the British Columbia mainland coast, but the feature is especially distinct on certain islands in the Great Bear Rainforest. For example, on Princess Royal Island, one in ten black bears is white, and on Gribbell Island, directly north of Princess Royal, it's one in three. It's unclear how the feature arose. One theory was the "glacial bear" hypothesis   that the spirit bear represented a remaining adaptation from the last great ice age, which ended here 11,000 years ago. At that time most of modern-day British Columbia was still icebound, and a white coat may have offered camouflage  . But the "glacial bear" theory raised a question: Why didn't the white fur feature die out when the glaciers  _ ? Researchers have recently proved that the spirit bear's white coat gives it an advantage when fishing. Although white and black bears tend to have the same success rate after dark, there is a difference during the daytime. White bears catch salmon in one-third of their attempts. Black individuals are successful only one-quarter of the time. "The salmon are less concerned about a white object as seen from below the surface," a scientist guesses. That may answer part of the question about why the white-fur feature continues to flourish today. If salmon are a coastal bear's primary fat and protein source, a successful female can feast on salmon to store more fat for winter, potentially increasing the number of cubs she can produce. From the passage we know that   _  .
to be born white, a bear's parents have to be white it's not natural for white bears to be born to black parents to be born white, a bear must inherit the mutation from both parents we know for sure that the spirit bear represented a remaining adaptation from the last great ice age
Question: With a wildlife guide, I stepped on a journey to find one of the rarest creatures in the world ----the spirit bear, a walking contradiction--a white black bear. Neither albino   nor polar bear, the spirit bear is a white variant   of the North American black bear, and it's found almost exclusively here in the Great Bear Rainforest. At 25,000 square miles ---- one and a half times as big as Switzerland--the region is a spooky , wild, mysterious place: there are wolves here that fish. Deer that swim. Western red cedar trees that have stood a thousand years. And a black bear that is white. Scientists know how black bears are born white. They're just not sure why. The phenomenon, known as Kermodism, is caused by a recessive mutation   at the MC1R gene, the same gene associated with red hair and fair skin in humans. To be born white, a bear must inherit the mutation from both parents. The parents themselves don't have to be white. They just need to carry the recessive mutation. So it's not uncommon for white bears to be born to black parents. White fur happens in only one of every 40 to 100 black bears on the British Columbia mainland coast, but the feature is especially distinct on certain islands in the Great Bear Rainforest. For example, on Princess Royal Island, one in ten black bears is white, and on Gribbell Island, directly north of Princess Royal, it's one in three. It's unclear how the feature arose. One theory was the "glacial bear" hypothesis   that the spirit bear represented a remaining adaptation from the last great ice age, which ended here 11,000 years ago. At that time most of modern-day British Columbia was still icebound, and a white coat may have offered camouflage  . But the "glacial bear" theory raised a question: Why didn't the white fur feature die out when the glaciers  _ ? Researchers have recently proved that the spirit bear's white coat gives it an advantage when fishing. Although white and black bears tend to have the same success rate after dark, there is a difference during the daytime. White bears catch salmon in one-third of their attempts. Black individuals are successful only one-quarter of the time. "The salmon are less concerned about a white object as seen from below the surface," a scientist guesses. That may answer part of the question about why the white-fur feature continues to flourish today. If salmon are a coastal bear's primary fat and protein source, a successful female can feast on salmon to store more fat for winter, potentially increasing the number of cubs she can produce. From the passage we know that   _  .
A. to be born white, a bear's parents have to be white
B. it's not natural for white bears to be born to black parents
C. to be born white, a bear must inherit the mutation from both parents
D. we know for sure that the spirit bear represented a remaining adaptation from the last great ice age
Answer: