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【单选题】

Walking exercises ____ be boring at times, but on a clear sunny morning nothing could be better.
[ ]

A.
may
B.
will
C.
must
D.
should
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参考答案:
参考解析:
.
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举一反三

【单选题】Madame Curie is believed _______ radium.

A.
discovering
B.
having discovered
C.
to have discovered
D.
to discover