单项选择题

Trial and error () the source of our knowledge.

A.is
B.are
C.has been
D.have been

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单项选择题

There are a lot of bitter stories () his success.

A.after
B.behind
C.under
D.down

单项选择题

I like this warm weather. It feels () spring.

A.like
B.as
C.like as
D.as like

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