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
网友回答
A. as
B. what
(答案→)C. which
D. who
解析：考查定语从句。Which指代先行词river在定语从句中做主语；故C正确。