Nobody in the family knows for sure why ________   [ ]

A. it broke out fierce quarrels between the newly wedded couple.   

B. the newly wedded couple often broke out fierce quarrels   

C. fierce quarrels often broke out between the newly wedded couple   

D. fierce quarrels were often broken out between the new wedded couple

比的前项和后项同时乘或除以______（0除外），______不变，这叫做______．