使用时可溶于轻质液状石蜡中喷于颗粒上的润滑剂是
A．聚乙二醇
B．氢化植物油
C．滑石粉
D．硬脂酸镁
E．月桂醇硫酸镁
参考答案：B
什么是齿轮泵？齿轮泵的构造如何？
Liu Mei often helps her mother _____ housework. [ ]
A. does
B. do
C. doing
D. to doing
