【单选题】

{{I}}Questions 14 to 16 are based on the following passage.At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage.{{/I}} The reason why birds don’t get lost on long flights ______.

A、has been known to scientists for years
B．has only recently been discovered
C．is known by everyone
D．will probably remain a mystery