Question

IM STUPID. Whats the difference between the rate and initial amount? Like for the question :
Julio takes his brother to a music store. The music store sells CDs for $15 each. If Julio and his brother have $50, how many CDs can they buy? ////////The answers choices are :

$15 = rate; $50 = initial amount

$15 = initial amount; $50 = rate

$15 = rate; -$50 = initial amount

$15 = initial amount; -$50 = rate