去分母得：3x-（9x-2）-12=0，
去括号得：3x-9x+2-12=0，
移项、合并得：-6x=10，
系数化为1得：x=-53．
数据库系统在三级模式之间提供了两层映射，这两层映射是______和模式/内模式映射。
It was in the school ____ he had studied ____ he began the important experiment.
A．that; where
B．in which; which
C．where; that
D．where; in which
