After an area has been hit by a natural disaster, there is often a great demand for plywood for repairing damaged homes. Retailers in the area often raise prices on new shipments of plywood to well above their predisaster prices, and some people denounce these retailers for taking advantage of a disaster to make more money on each sheet of plywood they sell. In fact, however, these retailers do not make more money on each sheet of plywood than before the disaster, because transporting the plywood into devastated areas is difficult and expensive, and ttherefore the plywood' s cost to retailers is higher than it was before the disaster. Which one of the following is an assumption on which the argument depends? ['The increase in the prices charged by retailers for plywood following a natural disaster does not exceed the increase in cost to those retailers.', 'Suppliers of plywood do not transport as much plywood to an area after it has been affected by a natural disaster as they did before it was so affected.', 'Retailers must pay the full amount of any increase in shipping costs.', 'No retailer makes enough money on each sheet of plywood sold to absorb for long an increase in shipping costs without raising prices.']