采用石膏抹灰时，石膏灰中不得掺用（）。
A、牛皮胶
B、硼砂
C、氯盐
D、107胶
参考答案：C
After three years of hard work, they at last _____ all the money they had borrowed.
A．paid off
B．paid back
C．paid in
D．paid for
水泵安装高度
