my aunt got her bachelor's degree at 19 and her master's deg

my aunt got her bachelor's degree at 19 and her master's degree at 21
--oh,she____a smart student
a.must be b.has always been c must have been d has to besuffering
答案是选c的,请问为什么呀 最好每选项都讲一下~
不好意思 D 是 had to be 看错了。