单项选择题


Passage 2

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
A．Teacher and student.B．Customer and waitress.C．Lawyer and client.D．Boss and secretary.
A．Teacher and student.
B．Customer and waitress.
C．Lawyer and client.
D．Boss and secretary.
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