单项选择题

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
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单项选择题
The conversation is about______.
A．the work of the future
B．the workplace of the future
C．the food of the future
D．the entertainment of the future
单项选择题
Mary will not be able to come to the birthday party as she is ______ with a cold.
A. laid out
B. laid up
C. laid by
D. laid down
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