A local animal shelter receives at least four cats and seven dogs a day. The shelter has the capacity to receive up to 30 cats and dogs a day. If the shelter can raise donations for the animals at a rate of 3 per cat and 5 per dog per day, how many cats and dogs should the shelter accept to maximize the amount of donations per day?
a) 10 cats, 20 dogs
b) 15 cats, 15 dogs
c) 20 cats, 10 dogs
d) 25 cats, 5 dogs