A:was painting
B:will be painting
C:have painted
D:have been painting
已知a,b∈R,x=a3﹣b,y=a2b﹣a,试比较x与y的大小.
如图,在Rt△ABC中,∠ACB=90°,D、E、F分别是AB、BC、CA的中点,若CD=3cm,则EF=____cm.
Mr. Brown is so strict ______ us that everyone in Grade 8 is scared _____ him.
细菌和真菌的培养通常包括以下步骤,请选择正确的排列顺序( )
①配制培养基 ②在适宜的温度下培养 ③接种 ④高温灭菌.