单项选择题

For years, doctors__________ millions of patients' lives with the help of microscopes.

A．have saved
B．are saving
C．will save
D．were saving
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单项选择题
Not until yesterday__________ anything about the project that will be completed soon.
A．did I learn
B．have I learnt
C．I learnt
D．that I learnt
单项选择题
It's my great honor__________ to give a speech at the opening ceremony.
A．to invite
B．inviting
C．having invited
D．to be invited
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