Peter doesn't smoke now, but he ________ for almost twenty years.
A:was smoking
B:had smoked
C:has smoked
D:smoked
这里的一草一木都有着许多动人的故事。
Even a flower or a tree here ____ moving stories.
动点Pa,b在区域x+y-2≤0x-y≥0y≥0上运动,则ω=a+b-3a-1 的范围( )。
下列与化学概念有关的说法错误的是( )