The american trucking association anticipates a shortage of trucker drivers in coming years as the number of retirees exceeds the number of new drivers who enter the profession. the number of train boxcars is also expected to fall by over 40 percent in coming years because federal regulations limit boxcars to 50 years of service and many currently in use will reach that limit by 2020. lumber and paper producers that depend on boxcars for their shipments fear that they will have to depend more on trucks for transportation, which will cost as much as 20 percent more than shipping by rail. sources: lindsay ellis and laura stevens, "international paper focuses on improving shipment planning, wall street journal, july 30, 2015; and bob tita, "why railroads can't keep enough boxcars in service," wall street journal, june 21, 2015. the reduction in the number of trucks and boxcars will

(a) likely increase transportation costs and shift the short-run aggregate supply curve to the left.
(b) likely affect the short-run aggregate supply curve because of the anticipated shortage of truck drivers.
(c) not likely affect the short-run aggregate supply curve because the change is related to prices, not wages.
(d) not likely affect the short-run aggregate supply curve because the change is related to regulations, not prices.