单项选择题

In this section, you will hear a monologue. The monologue will be read twice. After the monologue, there will be a one-minute pause. During the pause, read the questions and the four choices marked A, B, C and D, and decide which is the best answer. Then mark the corresponding letter on the answer sheet with a single line through the centre. Questions 16 to 20 are based on the passage you’ve just heard.
What evolutionary change in birds led to a nest’s building

A. Their flying ability improved greatly.
B. They became warm-blooded.
C. They began to lay eggs.
D. They live in cold places.