parts vs. slices

[Snypy]

English Language Tests, Intermediate level

ESL/EFL Test #181 "Facts about pizza in the US: pizza industry expressions", question 2

Americans eat 350 ......... of pizza per second.

(a) slices
(b) segments
(c) parts
(d) fragments

English Language Tests, Intermediate level

ESL/EFL Test #181 "Facts about pizza in the US: pizza industry expressions", answer 2

Americans eat 350 slices of pizza per second.

Correct answer: (a) slices

Your answer was: incorrect
Americans eat 350 parts of pizza per second.
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Hello,

Could you explain why I cannot use word 'parts' there? (I know that you can have slices of bread but not of pizza).

Thanks

[Mister Micawber]

Yes, you can have slices of pizza. You can have slices of pizza, cake, pie, ham, bread, meatloaf, cheese, lemon, roast beef, banana-- any food that can be cut thinly into sections. 'Parts' does not apply to any foods that I know of.
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Native English teacher at Mister Micawber's