单项选择题

_________ carefully if any change occurs when doing experiments in the lab.

A．Observe
B．To observe
C．Observed
D．Observing
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单项选择题
---Hi, let’s go skating.--- Sorry, I’m busy right now. I _________ in an application form for a new job.
A．fill
B．have filled
C．am filling
D．will fill
单项选择题
Some animals carry seeds from one place to another, _________ plants can spread to new places.
A．so
B．or
C．for
D．but
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