The government broadened land ownership by returning land that had been sold to large landowners in the late Ming period by families unable to pay the land tax. To give people more incentives to participate in the market, they reduced the tax burden in comparison with the late Ming, and replaced the corvée system with a head tax used to hire laborers. The administration of the Grand Canal was made more efficient, and transport opened to private merchants. A system of monitoring grain prices eliminated severe shortages, and enabled the price of rice to rise slowly and smoothly through the 18th century. Wary of the power of wealthy merchants, Qing rulers limited their trading licenses and usually refused them permission to open new mines, except in poor areas. These restrictions on domestic resource exploration, as well as on foreign trade, are held by some scholars as a cause of the Great Divergence, by which the Western world overtook China economically.