The reasons why birds don't get lost on their 10ng flights ________.
A.) have been known to scientists for years.
B.) have only recently been discovered
C.) are known by everyone
D.) will probably remain a mystery
正确答案是B
The reasons why birds don't get lost on their 10ng flights ________.
A.) have been known to scientists for years.
B.) have only recently been discovered
C.) are known by everyone
D.) will probably remain a mystery
正确答案是B