Mark Twain worked for several years as a pilot on a steamboat, taking passengers up and

down the Mississippi, the great river ___flows from the north of the US near the Canadianborder.

A．as

B．what

C．which

D．who

硅酸盐水泥的下列性质及应用中，哪一项是错误的？（）

A.水化放热量大，宜用于大体积混凝土工程

B.凝结硬化速度较快，抗冻性好，适用于冬季施工

C.强度等级较高，常用于重要结构中

D.含有较多的氢氧化钙，不宜用于有水压作用的工程