①一着不慎,满盘皆输
②一切皆流,无物常驻
③星星之火,可以燎原
④福祸相倚,物极必反
A:①③
B:①④
C:②③
D:②④
Our government has tried many ways to the smog(雾霾)problem. Maybe we'll have a clear sky in the near future.
已知函数fx=cosx+sinx,则f'π4=____ .
My mother often makes me ____ (help) her.