I'm writing a sentence and I cannot think of a good word to fit in.
...However, input hypothesis, despite its popularity and great contribution that it made, did not go without criticism. Noticing hypothesis and comprehensible output hypothesis were the main [the word] of it.
My first thought was to use critic, but it seems critic is used to describe a person. Can I use it here, too? If not, what word can I use there without being awkward?!
I should say that I can't use rival here either, I really hope I could, because these two theories propose that input hypothesis is necessary but not sufficient.