In this section, you will hear several passages. Listen to the passages carefully and then answer the questions that follow. Questions 11 to 14 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now, listen to the passage. What evolutionary change in birds led to nest building
A．Their flying ability improved greatly. B．They became warm-blooded. C．They began to lay eggs. D．They changed their migration patterns.