You visit a local Starbucks to buy a frap. The barista explains it comes in three sizes

You visit a local Starbucks to buy a frap. The barista explains it comes in three sizes, Tall, Grande, or a Venti. The prices are $3.50, $4.00, and $4.50 respectively. 

a) What are the variables and cases?
b) Which variable is the explanatory variable? Which is the response variable?
c) The Tall is 12 ounces, Grande is 16 ounces, and the Venti is 20 ounces. Answer parts a) and b) with ounces in place of names for sizes.