21. Frank ______stamps in his spare time. It’s his hobby.
A
is collecting
B
collects
C
collected
D
was collecting
关键词是It’s his hobby.， 可知此题是一般现在时，所以用collects。故选B.
本题是语法知识题。本句意思是：弗兰克在闲暇时间收集邮票。这是他的爱好。此处选B.
把握不准时态。