问题补充：
More than two years ago, people knew little about the universe.
A. thousands
B. thousand
C. thousands of
D. thousand of
答案：
A. thousands
(答案→)B. thousand
C. thousands of
D. thousand of
解析：此题考查名词的量。确切数量常用数词+hundred/thousand/million/billion+名词的复数形式；不确切数量常用hundreds/thousands/millions/billions + of+名词的复数形式.
