Which of the following statements is/are incorrect?

a. The headward extension of gullies can be controlled by gully plugging, terracing or by planting cover vegetation. b. In arid and semi-arid areas, cultivable lands can be saved from encroachment by sand dunes through developing shelter belts of trees and agro-forestry. c. Lands with a slope gradient of 15 - 25 percent should not be used for cultivation.