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？