题目

单项选择 There were many apples ______ the tree. A monkey was picking them ______ it. A. on; on B. in; in            C. on; in            D. in; on 答案：选C。考查介词的用法。树上生长的东西用on表示, 而非树上生长的东西则用in表示。2．下列关于育种的说法不正确的是（　　）A．古人通过汰劣留良来选择和积累优良基因B．杂种优势主要是利用杂种F2的优良性状C．高产青霉素菌株是通过诱变育种获得的D．质粒是拟核或细胞核外能自主复制的DNA