Why simple gerunds and complex gerunds are not parallel?

MGMAT says:

When looking at parallel elements in a list, concrete nouns and action nouns cannot be considered parallel. Whereas concrete nouns are specific things (e.g. rock, cup, year, tiger), action nouns refer to actions (the pollution of ..., the rotation of ...). Simple gerunds (typing, tying, eating, giving) are more like concrete nouns, whereas complex gerunds (the typing of ..., the typing of ..., the eating of ..., the giving)

Incorrect: I enjoy giving presents and the giving of food during the holidays.

Here, although "giving presents" and "the giving of food" are both nouns, the first is a concrete noun/simple gerund, while the latter is an action noun/complex gerund. Just reading the sentence, you can tell why this would not be parallel.

Correct: I enjoy the giving of presents and the giving of food during the holidays.
Correct: I enjoy giving presents and giving food during the holidays.
Correct: I enjoy giving presents and food during the holidays.

Note: There is a subtle difference in meaning between the first two correct sentences. The first one implies that he personally enjoys giving things during the holidays. The second implies that he enjoys the fact that presents and food are given.

Also, between the second and third, the third is better because it's more concise.

Hope that helps.

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