GMAT Sentence Correction (SC) | Re: Because chickens lack teeth, they need another way to break

Because chickens lack teeth, they need another way to break apart the food they eat before reaching the stomach, and for this reason, chickens have a gizzard in which stones they swallow are used to grind their food.

A)before reaching the stomach, and for this reason, chickens have a gizzard in which stones they swallow are used to grind their food.
B)before it reaches the stomach, and for this reason, chickens have a gizzard in which stones they have swallowed is used to grind their food.
C)before
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