Oils abandoned in sewers by restaurants, hotels and other units are picked up by illegal traders, processed into "edible oil" and sold to consumers, which is notorious.

A Aspergillus flavus is a multicellular fungus, parasitic and saprophytic. A is wrong;

B, "gutter oil" is a kind of non-edible oil with poor quality and unsanitary. Containing toxins, flowing into rivers will cause eutrophication of water bodies. Once eaten, it will destroy white blood cells and digestive tract mucosa, cause food poisoning and even cause cancer. B wrong;

C, fat is decomposed into glycerol and fatty acids in human small intestine, and C is correct;

D, fat contains more hydrogen than sugar, needs more oxygen to participate in the reaction and releases a lot of carbon dioxide, so fat generally does not provide energy. D error.

So choose: C.