Difference between "dumping", "discarding" and "disposing"

Could anybody help me, please? I'm having trouble solving this CAE exercise:

"The amounts of household waste produced anually are on the increase. As a result, discarding / disposing / dumping of such large amounts is becoming a serious problem."

I've always thought "discarding", "disposing" and "dumping" were sinonyms in this case. To my surprise, the answer key of this exercise gives "disposing" as the correct answer. I have checked a collocations dictionary, regular English-English dictionaries and looked for the difference between these words, but I still have no idea.

Does anybody know why "disposing" is the correct answer?

Thanks in advance.
Flávia

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These Cambridge tests are tricky, sometimes.

The focus here is not which verb is the most suitable, since there is very little difference in meaning (perhaps nothing at all) among them. You are overlooking the use of the preposition. Check below:

Dumping of such large amounts
Discarding of such large amounts
Disposing of such large amounts (you are required to use OF after dispose whereas you do not use it after dump or discard)

Well, this is the way I see it.

Best,

Marcelo.