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In the following fill-in-the-gap exercise:
Sometimes an allergy can disappear as ... as it arrived, without any treatment.
Would it be allright to use "fast"? The answer key only mentions "quickly/suddently". Thank you.
Sadly, in this case fast cannot be used. The word you are looking for describes disappear, which is a verb. This means, you have to use an adverb.
Fast is a verb, so it cannot be used here.
As the answer key says, quickly is probably the best answer to use here, as it is an adverb.
...
Fast is a verb, so it cannot be used here.
...
Hmm, I am absolutely certain that the asker is aware of the fact that fast is also a verb. The thing is he wanted to knw why fast (the adverb) couldn't be used as one of the possible choices. That's the case, I think, Niall.
Fast vs quickly 
Re: Fast vs quickly 
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