4. Michelle is buying pieces of lumber and boxes of nails at a hardware store. The expression below represents the total cost of xpieces of lumber and of nails. What do the coefficients in the expression represent?
A. The total cost for all of the items Michelle buys
B. The number of pieces of lumber and the number of boxes of nails
C. The total number of items Michelle buys
D. The cost of each piece of lumber and the cost of each box of nails