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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