单项选择题

Scientists employ many modern techniques to ______ the origin of the world.

A．watch
B．grasp
C．treat
D．probe
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单项选择题
I'd like to ______ this old ear for a new model but I can't afford it.
A．interchange
B．exchange
C．replace
D．convert
单项选择题
Every word processor we sell comes with a two year ______.
A．assurance
B．protection
C．security
D．guarantee
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