GMAT Sentence Correction (SC) | Re: Salsa Dancers warm up before every performance by doing a

Hi,
I will try to explain what is wrong with b

Salsa Dancers warm up before every performance by doing a series of warmup and stretching exercises,and it reduces the chance of injury.

(B) exercises, which reduces

which here refers to 'exercises'
1. if we see grammatically then plural noun + singular verb(reduces) wrong.
2. If we understand
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