顶交法
参考答案：选育一个遗传基础广泛的品种群体作为测验中，来测定自交系的配合力。
Remember never to ask a European and an American personal questions. It is almost certain that
they ______ for it . [ ]
A. are not prepared
B. don't prepare
C. haven't prepared
D. are not preparing
闭塞设备有几种？它的任务是什么？
