题目内容
Exactly when the first people arrived in __1__ as California, no one really knows.
Scientists believe that these first setters crossed the Berlin Strait from Asia to Alaska __2__ of a land bridge.
Thousands were killed or __3__ slavery.__4__, many __5__ brought by Europeans.
Cloning has two __6__.Firstly, Secondly, it is valuable for research on __7__ and for medical research on animals.Many __8__ to clone mammals failed. But at last the __9__ of the scientists was rewarded in 1996 __10__— the cloning of Dolly the sheep.
1.what we now know 2.by means of 3.forced into 4.In addition 5.died from 6.major uses 7.new plant species 8.attempts 9.determination and patience 10.with a breakthrough
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