A/the

A rat is bigger than a mouse, and lives in sewers and cellars.

lucas21cCould you tell me which one is right between (A1) and (A2), between (B1) and (B2) in the following sentence?[ (A1) A rat / (A2) The rat ] is bigger than [ (B1) a / (B2) the ] mouse, and lives in sewers and cellars.

Unless you're talking about a rat and a mouse that you know personally (perhaps as your pets?), it's better if you use the version given ab

Then why do I have to use the definite article, 'the', in these sentences, "The giraffe is the tallest among all animals" and "The dog is the most faithful of all animals"? The 'giraffe' and 'dog' here do not mean the animals that I have met personally before, and they just represent the species of the whole dogs and giraffes respectively.

Let me refer you to a thread from a couple of years ago that may help:

"THE" tiger is a strong animal.?

tamguatlayA rat is bigger than a mouse, and lives in sewers and cellars.

Actually, this sounds a bit odd to me. "A rat is bigger than a mouse" would be OK as a generic statement, but when the next part is added, it sounds to me as if you're saying that one individual rat lives in sewers and cellars.