【单选题】

{{I}}Questions 17-20 are based on the following monologue about the animals’ migration. You now have 20 seconds to read questions 17-20.{{/I}} Birds may use their eyes to measure

A、changes in temperature to help them migrate.
B．the position of the sun to help them migrate.
C．periods of light to help them migrate.
D．the distance they have traveled to help them migrate.