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

In the original sentence, the underlined portion has the followingerrors:
before reaching the stomach , and for this reason, chickens have a gizzard
in which stonesthey swallow are used to grind their food.

Using 'it' makes the antecedent clearly refer to food.
Also, the stones they have swallowed are used to grind their food gives the meaning clarity.

This when replaced with :
before it reaches the stomach, and for this reason, chickens have a gizzard in which stones they have swallowed are
...

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