问题补充：
Collecting stamps as a hobby increasingly popular during the past fifty years.
A. becomes
B. became
C. has become
D. had become
答案：
A. becomes
B. became
(答案→)C. has become
D. had become
解析：主语collecting stamps as a hobby被看成是整体，谓语动词用第三人称单数形式。而时间状语during the part fifty years限制了句子用完成时态。
【标签】动名词作主语
