国防按性质和建设目标可以分为扩张型国防,中立型国防,()和自卫型国防。
A、民主型国防
B、联盟型国防
C、和平型国防
D、进攻型国防
参考答案:B
I ______ him in time, otherwise he would have fallen off the bike.
A.had supported
B.supported
C.have supported
D.Support
简述致突变作用的不良后果。