What does "piles" mean?

[Membra]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #595 "Real Life: Hiking and The Great Outdoors (1)", question 6

My mother-in-law lives near the ......... in Nevada. This area usually refers to low-lying areas directly by or in the vicinity of the bottom area or foot of a mountain range.

(a) piles
(b) heaps
(c) foothills
(d) peaks

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #595 "Real Life: Hiking and The Great Outdoors (1)", answer 6

My mother-in-law lives near the foothills in Nevada. This area usually refers to low-lying areas directly by or in the vicinity of the bottom area or foot of a mountain range.

Correct answer: (c) foothills
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what do "piles" mean?

Membra

[Mister Micawber]

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Look up the singular, 'pile', in your dictionary. None of its meanings is appropriate in this sentence.
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Native English teacher at Mister Micawber's