Which of the following statements is/are correct?

a. The Himalayan yew is under great threat due to over-exploitation because of the chemical compound taxol that is extracted from the tree and is used as a cancer drug. b. Many foresters and environmentalists hold the view that the greatest degrading factors behind the depletion of forest resources are grazing and fuel-wood collection. c. Substantial parts of the tribal belts, especially in the northeastern and central India, have been deforested or degraded by shifting cultivation.