This is _____ useful book. I've read it for _____ hour.
A. an, /
B. an, an
C. a, an
D. an, the
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The teacher told his classlight travels faster than sound.
The man ______ (who/what) you saw yesterday is a doctor from Beijing.
Here is a book ______(who/that) will give you a lot of usful information.
The girl ______(who/which) sang well at the party last night is my friend.