压裂中前置液的作用，主要是疏通地层，制造裂缝。（）
参考答案：对
Although he has not gone to college，he shouldn't ________ studying for his degrees.
A．be prevented
B．be stopped
C．be kept from
D．be protected from
下列不属于烟气分析的主要项目的是（）。
A.二氧化碳
B.氧
C.一氧化碳
D.水
