Polar bears live mostly on _____ sea ice, which they use as ___ platform for hunting seals. A. a; a B. a; the C. 不填;a D. the; 不填 答案:C 解析: sea ice 海冰, 是固定结构,不用冠词;platform平台,是可数名词。一体细胞基因型为AA,经过某一过程这个细胞的基因型变为AAAA,这个过程最可能是 ( )A.染色体数目变异B.DNA复制C.交叉互换D.自由组合