问题补充：
It was the building of the rail network from the west to the east coast brought even larger numbers to California in the 1860s.
A. that
B. what
C. how
D. which
答案：
(答案→)A. that
B. what
C. how
D. which
解析：考查强调句型。强调句型的基本结构为：it is/was 被强调成分that/who 其它成分；其最大的特点是去掉it is/was that/who句子仍然是成立的。本题强调主语the building of the rail network from the west to the east coast。句意：在19世纪60年代，正是从西部到东部沿海的铁路线把更多的人带到了加利福尼亚。
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