单项选择题


A．defend
B．maintain
C．keep on
D．safeguard
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单项选择题
A．to studyB．studyingC．studiedD．being studied
A．to study
B．studying
C．studied
D．being studied
单项选择题
A．whichB．thatC．theyD．what
A．which
B．that
C．they
D．what
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