Sentence Correction (SC) | Re: Glaciers form when the snow, sleet, and hail that fall in a

Based on the comparison, I know C is the only correct answer, yet I am still confused about what falls in the region. THE AMOUNT or SNOW, SLEET AND HAIL? According to my knowledge, the answer is SNOW, SLEET AND HAIL - snow, sleet and hail, not the amount, falls in the region. Because of the "fallS", snow, sleet and hail is thought to be singular; can I view "snow sleet and hail" as a set, like a knife and fork.

Additionally, a friend of mine faced the problem of "snow
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