I have a statement with an error, and I don't understand the explanation given to me about what is wrong with it.
A letter of recommendation b)/ from the principal and c)/ the Head of English department have helped him to d)/ get this lucrative job. e)/ No error
The answer for the question about the above sentence shows we have to use has instead of have: have helped him to should be has helped him to. This is what I don't understand.
My question is that has is used to represent the singular form, so why do we have to use has? In the above sentence, "the principal and Head of English department" denotes two people — so why do we have to use has?