I am studying the English grammar for an exam, and the book tells me what I thought was right is actually wrong. So I was wondering if I could get some help here.
In the sentence 'Prime numbers are what are left when you have taken all the patterns out.', why should are in bold be is? So that it is 'Prime numbers are what is left when...'
I thought it was correct already since the subject is in plural.
Thanks in advance!