They always do _______ activities _______.
A:after-school; after school
B:after-school; after-school
C:after school; after school
D:after school; after-school
①方程fg(x)=0有且仅有三个解;
②方程gf(x)=0有且仅有三个解;
③方程ff(x)=0有且仅有九个解;
④方程gg(x)=0有且仅有一个解.
那么,其中正确命题是( )
已知(a﹣x)5=a0+a1x+a2x2+••+a5x5,若a2=80,则a0+a1+a2+••+a5=( )
如图所示,属于增大摩擦的是( )
海洋水族馆中,表演者需携带氧气瓶在水中与鱼“共舞”.氧气被压缩在钢瓶中,其原因正确的是( )