The teacher _______some old friends on his vacation trip to Paris.

A．dropped by

B．dropped in

C．droped by

D．droped in

后掠翼的缺点之一是（）.

A.翼根失速早于翼尖小翼部分

B.翼尖小翼失速早于翼根部分

C.当压力中心向前移动时，产生严重的机头下沉力矩