Nathan and Steven stacked boxes on a shelf,Nathan lifted 13 boxes and Steven lifted 15 boxes.the boxes that Nathan lifted each weighed 12 lb more than the boxes Steven lifted.let's represent the weight of the boxes that Steven lifted what expression represents the total number of pounds Nathan lifted? a.15(s + 12) b. s + 12 c.12(s + 13) d.13(s + 12)