Barbara buys a box of pens for $4. For every additional box she buys, she gets a $1 discount. Which expression represents the total cost of the pens, c, as a function of the number of boxes, b? c = f(b) = 4b – 1 c = f(b) = 4b + 1 c = f(b) = 3b + 4 c = f(b) = 4b + 3 c = f(b) = 4 + 3(b – 1)