无人机系统飞行器平台主要使用的是()。
参考答案:重于空气的动力驱动的航空器
用来加工外螺纹的工具叫()。
______ a little more time to think, he might have acted more sensibly.
A.If he took
B.If he has taken
C.had he taken
D.Should he take