______ scientists have learned a lot about the universe, there is much we still don't know.

1. ______ scientists have learned a lot about the universe, there is much we still don't know.

A. Once B. Since C. Though D. Unless

【考点】

连词; 引导让步状语从句的连词; 让步状语从句;

【答案】

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单选题普通

1. The girl thought her boyfriend had bought her a necklace of great value ________ in fact it was made of glass.

A. when B. while C. that D. as

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2. ________ I say Clancy is a smart boy, he still needs to work hard to achieve his goal.

A. If B. When C. As D. While

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3. I think Tom will make a good monitor, I'd like to vote for him.

A. but B. for C. or D. so

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