The rate at which a road wears depends on various factors, including climate, amount of traffic, and the size and weight of the vehicles using it. The only land transportation to Rittland's seaport is via a divided highway, one side carrying traffic to the seaport and one carrying traffic away from it. The side leading to the seaport has worn faster, even though each side has carried virtually the same amount of traffic, consisting mainly of large trucks. Which of the following, if true, most helps to explain the difference in the rate of wear? ["Wear on the side of the highway that leads to Rittland's seaport has encouraged people to take buses to the seaport rather than driving there in their own automobiles.", "A greater tonnage of goods is exported from Rittland's seaport than is imported through it.", "The volume of traffic to and from Rittland's seaport has increased beyond the intended capacity of the highway that serves it.", "All of Rittland's automobiles are imported by ship."]