采煤工作面支护的目的是什么？
参考答案：
保证工作面正常和安全地进行生产活动。
多项式x2y-x3y2-1+y4是______次______项式，其中常数项是______．
My sister studied hard at school when she was young, which contributes ________ her success in her career.
A．with
B．for
C．from
D．to
