Why is it wrong to say "quicker" in this sentence?

Our pet rabbit Kermit can eat quicker than both of our cats. But my teacher says it would be "quickly", not "quicker". Why? I think quicker works perfectly fine. For example ... I can say, "I'm stronger than James" .... would it make sense to say, "I'm strongly than... show more Our pet rabbit Kermit can eat quicker than both of our cats.

But my teacher says it would be "quickly", not "quicker". Why? I think quicker works perfectly fine.

For example ... I can say, "I'm stronger than James" .... would it make sense to say, "I'm strongly than James????"
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