We'll stay at this school for ____ six months .

选D
因为A,B,C意思上都合理。
another six months = six more months(再6个月)
at least six months 至少6个月
over six months = more than six months （6个多月）

C因为for 后不加B A是句意上的错误 C是超过的意思

选D吧。
A;another
句子意为：再呆6个月
B;at least
句子意为：至少呆6个月
C;over
句子意为：呆6个月以上

都对的。

C months是复数,因该用over,是超过的意思