生态平衡是一种( )
A:绝对平衡
B:动态平衡
C:永久的平衡
D:动植物数量相等
名著阅读
—Has Jack finished painting the picture yet?—I've no idea. But he______ it the whole afternoon.
若a→,b→为单位向量,则下面各式中,正确的是( )