对电梯进行清洁，就是单纯为了卫生，为了好看。（）
参考答案：对
Our family were thinking about __________ Hainan Island at this time yesterday.
[ ]
A. to visit
B. Visited
C. visiting
D. visit
钻井液密度随钻井液中（）的增加而增大。
A、固相含量
B、液相体积
C、粘度
D、pH值
