Not necessary, because even if babies DO pay extra attention to phrases with simple grammatical structures, we still know that children taught without simplified language master grammatical structures just as well and as quickly as those who aren’t taught with those methods. What matters is the efficacy of simple phrases in helping one learn grammatical structure and language, not whether they draw more attention from babies.

Not necessary, because the author’s conclusion is only that simplified language doesn’t help; that doesn’t require the author to think simplified language hurts language learning.

Necessary, because if it were not true — if some children who have mastered the grammatical structure of a language have NOT learned the language — then the fact that simplified language does not seem to improve mastery of grammatical structure would not establish that it doesn’t help one learn language. The negation of (C) opens the possibility that there are other ways simplified language could benefit learning of language even if it doesn’t help mastery of grammatical structure.

Not necessary, because what many linguists believe is irrelevant. The content of linguists’ belief has no effect on the reasoning, because thea argument isn’t based on what linguists believe.

Not necessary, because (E) arguably weakens if assumed. (E) asserts that there’s something else required in order to learn a language — that opens the possibility simplified language might help with that other aspect of language learning, even if it doesn’t help with mastery of grammatical structure.