A:spent; must have bee
B:cost; ought to have spent
C:wasted; wouldn’t have bee
D:took; can’t have spent
“龟鹤同池,龟鹤共100只,共有脚350只,问龟鹤各多少只?”设龟有x只,鹤有y只,则下列方程组中正确的是( )
人体口腔上皮细胞和番茄果肉细胞都具有的,能控制物质进出细胞的结构是( )
下列离子能在pH = 12的水溶液中大量共存的是