若单项式3x2by与2x4ya+1的和仍是一个单项式，则ab的值为 [ ]
A．2
B．0
C．-2
D．-4
答案：B
The story took place in Europe in the days before automobiles __ everyone used horses.
A．which
B．when
C．where
D．as
铸造生产的粘土分为（）。
