For several centuries there have been hairless dogs in western Mexico and in coastal Peru. It is very unlikely that a trait as rare as hairlessness emerged on two separate occasions. Since the dogs have never existed in the wild, and the vast mountainous jungle separating these two regions would have made overland travel between them extremely difficult centuries ago, the dogs must have been transported from one of these regions to the other by boat, probably during trading expeditions. Which one of the following is an assumption that the argument requires? ['Hairless dogs have never been found anywhere except in the regions of western Mexico and coastal Peru.', 'Centuries ago, it was easier to travel by boat between western Mexico and coastal Peru than to travel by an overland route.', 'If hairless dogs were at one time transported between western Mexico and coastal Peru by boat, they were traded in exchange for other goods.', 'Most of the trade goods that came into western Mexico centuries ago were transported by boat.']