俺来试试

B2 should be right.
Same as pants.

A pair of pants consists of two parts, so does a pair of sunglasses.

e.g.
My mother bought me a pair of new pants. They look great on me.
Or
my mother bought me a pair of new pants, which looks great on me.

If there is "a pair of" in sentence, singular form, otherwise plural form.

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