单项选择题

As early as in 1710 the iron industry in England complained of increasing competition from the American colonies. The American iron industry developed rapidly from that date until, by 1750, numerous furnaces, forges and mills were in operation in New England, the middle colonies, and Virginia. When large quantities of pig iron from the American colonies first entered England in 1735, the product proved to be such excellent quality that England iron makers became involved in a hitter argument over the future of the colonial iron industry. The English smelters, who changed native English iron ore into pig iron, insisted that American pig iron be kept out of England by means of high import taxes and, in fact, that the whole colonial iron industry be suppressed. In agreement with the iron smelters were owners of English mines and even forests, whose wood was used to fuel the furnaces which smelted the iron ore. On the other side of the issue were the English iron manufacturers who desired more cheap pig iron to make into nails, tools and other iron wares. The iron manufacturers therefore encouraged the production of pig iron in the American colonies. They wanted it to enter England tax-free, but, at the same time, demanded that the colonists be prevented from working their crude iron into finished products. In addition to the iron manufactures, English merchant ship owners were in favor of receiving American pig iron, for they looked forward to transporting the crude iron from America to England and the manufactured iron products from England to the colonies. The English wool industry supported the iron manufacturers, also, in the belief that the Americans would use the money received for shipments of crude iron to buy cloth made in England, thus discouraging the growth of wool manufacturing in America. The main topic of this passage is the _______.