问题补充：
people are standing outside the office. people there is at least over 1,000.
A. A number of; A number of
B. The number of; The number of
C. A number of; The number of
D. The number of; A number of
答案：
A. A number of; A number of
B. The number of; The number of
(答案→)C. A number of; The number of
D. The number of; A number of
解析：句意：许多人正站在办公室的外面。这里的人数至少超过1，000。两者后都可以跟可数名词的复数，意思和用法不同。a number of 许多，若干；the number of…的数目。故选C。
