单项选择题

Trees are a renewable resource that when managed properly can ________our needs indefinitely.

A．sustain
B．retain
C．maintain
D．obtain
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单项选择题
It is well-known that the retired workers in our country are to ________free medical care.
A．entitled
B．granted
C．presented
D．promised
单项选择题
It is laid down that undergraduate students have no ________to the rare book in the school library.
A．access
B．entrance
C．way
D．path
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