Which interrogative pronoun should replace the blank space in the following sentence?: ___ threw the football?
The missing word is who or what.
To find out the missing interrogative pronoun first let's think of the sentence as if it was not a question.
If you were to fill in the missing word in an indicative sentence the missing part would be the subject, because this statement has a verb (threw) and an object (the football), so if in the indicative sentence a subject is missing.
In the interrogative sentence we would have to ask about the subject using who (or rarely what if the subject was an animal or a machine)