12. Mr Black has two sons, ________ are
题型：选择题
问题：
12. Mr Black has two sons, ________ are fond of collecting stamps.
A．all of whom
B．all of them
C．both of them
D．both of whom
12. Mr Black has two sons, ________ are fond of collecting stamps.
A．all of whom
B．all of them
C．both of them
D．both of whom