阅读下列短文，从每题所给的A、B、C和D项中，选出最佳选项。

                                                                              Musicians wanted

      We want four good musicians for our rock band(摇滚乐队). Are you a musician? Can you sing? Can you play the piano, the guitar or the drums? Do you want to be the best musician? If you want to join us, please send(发送) an e-mail. Our e-mail address is rockband2009 @pep.com.cn. Or you can call Mr. Liu at 790-4566.

                                                                             Teachers needed

      Are you an art te acher? Can you paint (绘画) or draw well? Are you good with kids? We need two teachers for our art club. Please find Mrs Wang for more information(信息) or call 628-1183.

教室里有7人。下课了，老师走出教室，教室里有几个小朋友？

用³²P标记了玉米体细胞（含20条染色体）的DNA分子双链，再将这些细胞转入不含³²P的培养基中培养，则第二次细胞分裂的后期细中（　　）