Let me understand: why do you think both work in the sentence?
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
louiST If she knew the answer why both were correct, she would not be asking.louiST,
CalifJimYour English is really getting good!That's a perfectly constructed second conditional.REALLY???
norwolfMy English teacher from China told me: what = the…which, meaning it has something to do with the structure, because furnace fuel needs an article while is allowed needs a subject.Do you agree with him?Yes. That's an acceptable paraphrase, and it's sufficient if you don't want to get into the matter of usage, which I told you about above.