问题补充：
It was only after World War II had drawn to a close in the United States.
A. did the baby boom begin
B. that the baby boom began
C. when the baby boom began
D. then the baby boom began
答案：
A. did the baby boom begin
(答案→)B. that the baby boom began
C. when the baby boom began
D. then the baby boom began
解析：：考查强调句型。本题考查强调句型：it is/was被强调成分that/who…；强调句型的最大特点是把it is/was…that/who…去掉句子仍然成立。本题强调的是主语only after World War II had drawn to a close句意：只有在第二次世界大战结束后，美国人口出生快速增才开始。故B正确。
