Being able to speak another language fluently is a great________when you are looking for a job.[ ]

A. assistance  

B. chance

C. advantage  

D. importance

一个等比数列前几项和S_n=abⁿ+c，a≠0，b≠0且b≠1，a、b、c为常数，那么a，b，c必须满足（）。

A.a+b=0

B.c+b=0

C.a+c=0

D.a+b+c=0