Quick English question (about incorrect line)

A(Rarely) B(I have) been C(on) D(an) empty bus in San Francisco.

Which word is wrong?
Rarely, I have, on, or an??
What should it be?

It's for my English homework... Is it against the rules of this comm hereXP?

Now that I think about it, the problem with me is that I always tried to evade problem and try to make new lines instead of correcting it0_0