1.Ever since Dolly,the sheep ___in 1997,many scientists have

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1.Ever since Dolly,the sheep ___in 1997,many scientists have been working on cloning a human being.
A had cloned B has been cloned Cwould cloned D was cloned
2.The theory ___ about in 1514,but the csirntist only published it just defore his death in 1543.
Ahad put forward Bwas put forward
3.In American English,the letter"u" in some words,such as colour,___
Ais dropped Bare dropped
正确答案是D B A 但理由是什么？

1D ever since 后面的从句用一般过去时
2B but 连接的是两个并列的分句 ,所以but 前后连接的两个句子的时态应该一致,所以选B 用过去时的被动语态
3A 句子的主语应该是the letter"u" 所以谓语动词要用单数选B

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