So this is still a mystery to me. Can someone help me please? This is the question:
It was only when electric motors ______________ to become portable that vacuum cleaners became common household items.
1) became advanced sufficiently
2) sufficiently became advanced
3) had sufficiently become advanced
4) had become sufficiently advanced
And please tell me why the 3 wrong answers are wrong. Also, can we put an adverb like "sufficiently" between "had" and "become"? Does the answer to the last question only apply to this sentence or is it always like that? And one last thing, please choose one of the answers given. This was a test, so no other answers were available.