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
