单项选择题

This book has been in the works so long that I have lost ______ of most of the sources found for me by the staff of the library.

A．trace
B．trail
C．track
D．touch
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单项选择题
You ______ that letter to James. However, you didn't.
A．ought to write
B．ought to have written
C．should write
D．should be writing
单项选择题
______ there is little we can do to modify the weather, we can at least know what kind of weather to expect.
A．Since
B．When
C．While
D．Unless
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