a vet's office offers boarding for cats and dogs. boarding a cat cost$20 per night and a dog cost $25 per night if the vet's office boards 18 total animals in one night and earns a total of $400 which equations can be used to find the number of cats, x and dogs, y that were boarded?

a. x-y=18
20x-25y=40

b. x+y=18
20x+25y=400

c. x-y=18
25x-20y=400

d. x+y=18
25x+20y=400​