问题补充：
—Who is the man by a group of students there?
—A professor research on physics.
A. followed; did
B. following; did
C. followed; doing
D. following; doing
答案：
A. followed; did
B. following; did
(答案→)C. followed; doing
D. following; doing
解析：句意为：----后面跟着一群学生的那位男士是谁？ ----做物理研究的一位教授。the man 与follow之间的关系为被动作后置定语，应为followed, a professor与do 之间的关系为主动，教授做研究。故应为C
