Penguins live together,but each pair has a little piece of ground of their own.When a penguin wants to walk through its neighbor's ground,it must ask permission.If it does not do that,it will have to fight.Most of the time,penguins live on the water.They eat shell fish and look after their children carefully. All penguins are good parents--the male penguins are perhaps the best parents in the world. They walk in the sea in the middle of the dark Antarctic water. They choose their wives in the dark.They can only hear them--not see them.Then the female penguins lay their eggs and go away for about two months.The males take care of the eggs.If the eggs get cold,there will be no chicks.There is no food.The snow falls heavily and the wind blows strongly--sometimes at 150 kilometers an hour.The penguins do not move.When the females return from the sea,they will not remember their husbands.It does not matter.Only one thing matters--the eggs.Male penguins never fight--unless a penguin leaves a chick for a minute.They then fight because they all want it.They are strange and wonderful birds. It seems interesting and unusual that the male penguins choose their wives   _  .
when the female penguins return from the sea just by hearing in the darkness each time they see the lovely females after they have had their own grounds
Question: Penguins live together,but each pair has a little piece of ground of their own.When a penguin wants to walk through its neighbor's ground,it must ask permission.If it does not do that,it will have to fight.Most of the time,penguins live on the water.They eat shell fish and look after their children carefully. All penguins are good parents--the male penguins are perhaps the best parents in the world. They walk in the sea in the middle of the dark Antarctic water. They choose their wives in the dark.They can only hear them--not see them.Then the female penguins lay their eggs and go away for about two months.The males take care of the eggs.If the eggs get cold,there will be no chicks.There is no food.The snow falls heavily and the wind blows strongly--sometimes at 150 kilometers an hour.The penguins do not move.When the females return from the sea,they will not remember their husbands.It does not matter.Only one thing matters--the eggs.Male penguins never fight--unless a penguin leaves a chick for a minute.They then fight because they all want it.They are strange and wonderful birds. It seems interesting and unusual that the male penguins choose their wives   _  .
A. when the female penguins return from the sea
B. just by hearing in the darkness
C. each time they see the lovely females
D. after they have had their own grounds
Answer: