— __________ are singers. 

— My father is _____. [ ]

A. Tom and Jim's fathers; a singer and an actor

B. Tom and Jim's father; a singer and actor

C. Tom's and Jim's father; a singer and an actor

D. Tom's and Jim's fathers; a singer and actor

什么是完全失磁？造成完全失磁的原因有哪些？