B. “Raining cats and dogs” is an expression that means “heavy rainfall”
Anytime you look at a sentence to see if it’s grammatically correct, make sure to speak it out loud (if you can) to see if it would make sense. Plus, if you see “a” next to a word that starts will a vowel (ex: “hey that’s a apple!”) it is grammatically incorrect. Instead it should have “an” before something. (Ex: hey, that’s an apple!”