单项选择题

Adults sometimes envy children who play all the time （） they themselves have to work, rain or shine.

A. while
B. once
C. since
D. though

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单项选择题

A person with the blood type AB can （） any type of blood.

A. apply
B. recognize
C. receive
D. absorb

单项选择题

I（）come yesterday, but I could not.

A. was to have
B. must
C. ought
D. have to

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