主起落架转弯系统的主要作用（）
A、避免刮擦轮胎
B、减小转弯半径
C、减小操纵飞机转弯的力
D、控制飞机滑行方向
参考答案：D
z=f(u，x，y)，u=x2ey，其中f具有连续的二阶偏导数，则
=______．
Beijing a lot in the past ten years. Now it has become one of the biggest
modern in the world.
[ ]
A. changes, cities
B. changed ;city
C. has changed; cities
D. had changed; cities
=______．