桨叶对称翼型选择的原则是具有稳定的（）
参考答案：压力中心
At that time I had no idea ________ I could hand it to him without being seen.
A．if
B．how
C．which
D．that how
推焦执行系数是（）。
A.K1
B.K2
C.K3
