喜树碱主要通过特异性抑制DNA拓扑异构酶Ⅱ，破坏DNA合成，使肿瘤细胞死亡。
参考答案：错
The difficulty which we had _______ with the local people has now been removed·
A．communication
B．to communicate
C．communicated
D．communicating
采用持续训练法发展运动员的体能时，应注意练习的时间越长负荷的强度越大。 ( )
