My classmates and I like our English teacher.She is ( ) great fun.A.a B.an C.the D./

My classmates and I like our English teacher.She is ( ) great fun.A.a B.an C.the D./
My classmates and I like our English teacher.She is ( ) great fun.
A.a B.an C.the D./
应该选哪个?说清理由,并把关于这个句子的重点找出来.