题目

Generally speaking,          according to the directions, the drug has no side effect. A．when taking                                               B．when taken          C．when to take                               D．when to be taken 答案：B根据发现的古人类化石研究，人类进化史中关键的一种阶段是（　　）A．南方古猿B．能人C．直立人D．智人