问题补充：
.
It was his mother came in to prepare his lessons.
A. not until; did the boy begin
B. until; that the boy didn’t begin
C. until; did the boy not begin
D. not until; that the boy began
答案：
A. not until; did the boy begin
B. until; that the boy didn’t begin
C. until; did the boy not begin
(答案→)D. not until; that the boy began
解析：not until 的强调句。写成It was not until his mother came in that ....
