Thousands of puffins  live in Maine and on islands in the Gulf of Maine. But the puffins may be in danger. Last summer, the percentage of laid eggs that successfully produced baby puffins took a dive. Scientists also found a decline in the average body weight of the adult and baby puffins on Machias Seal Island, home to the area's largest colony. Over the winter, dozens of the seabirds from the region were found dead, likely from starvation. What's causing the puffin trouble? Scientists think it may be a shortage of food. With ocean temperatures rising, fish populations have moved around. Normally, puffins' primary food source is herring, a type of fish. A lack of herring in the area could be causing the problem. Butterfish from the south have become more abundant in the Gulf of Maine and could be a new food source for birds. But Steve Kress says butterfish may be too big and round for baby puffins to swallow. Puffins spend most of their lives at sea. They come ashore to breed each spring and return to the ocean in August. The chicks swim to sea about 40 days after hatching. Puffin populations stretch across the North Atlantic, from Maine to northern Russia. Maine's puffin population has been at risk in the past. In the 1800s, they were hunted for their food, eggs and feathers. By 1901, only one pair of puffins remained in the state. Thanks to the help of local lighthouse keepers and seabird restoration programs, the state's puffin population has been restored to more than 2,000 birds. Scientists aren't sure what will happen to the Gulf of Maine's puffins. The birds may move further north. Kress says he hopes the Gulf population will sustain itself and then he continued. "You never know what climate change will bring," Kress said. "Historically fish could move out and more southerly fish could move in, and puffins may adapt to the new fish. Only they will know how the story will unfold." In the scientists' opinion,   _   is contributing to the puffin trouble.
a lack of herring environmental pollution the increase of birds the huge size of butterfish
Question: Thousands of puffins  live in Maine and on islands in the Gulf of Maine. But the puffins may be in danger. Last summer, the percentage of laid eggs that successfully produced baby puffins took a dive. Scientists also found a decline in the average body weight of the adult and baby puffins on Machias Seal Island, home to the area's largest colony. Over the winter, dozens of the seabirds from the region were found dead, likely from starvation. What's causing the puffin trouble? Scientists think it may be a shortage of food. With ocean temperatures rising, fish populations have moved around. Normally, puffins' primary food source is herring, a type of fish. A lack of herring in the area could be causing the problem. Butterfish from the south have become more abundant in the Gulf of Maine and could be a new food source for birds. But Steve Kress says butterfish may be too big and round for baby puffins to swallow. Puffins spend most of their lives at sea. They come ashore to breed each spring and return to the ocean in August. The chicks swim to sea about 40 days after hatching. Puffin populations stretch across the North Atlantic, from Maine to northern Russia. Maine's puffin population has been at risk in the past. In the 1800s, they were hunted for their food, eggs and feathers. By 1901, only one pair of puffins remained in the state. Thanks to the help of local lighthouse keepers and seabird restoration programs, the state's puffin population has been restored to more than 2,000 birds. Scientists aren't sure what will happen to the Gulf of Maine's puffins. The birds may move further north. Kress says he hopes the Gulf population will sustain itself and then he continued. "You never know what climate change will bring," Kress said. "Historically fish could move out and more southerly fish could move in, and puffins may adapt to the new fish. Only they will know how the story will unfold." In the scientists' opinion,   _   is contributing to the puffin trouble.
A. a lack of herring
B. environmental pollution
C. the increase of birds
D. the huge size of butterfish
Answer: