The correct option is A Both A and R are true and R explains A
When alpha particles enter the body accidentally by ingestion or inhalation, they release all their energy in a very small area because of their low penetration power. Thus,energy released by the particles will be imparted to a particular organ rather than distributed to a larger area around it. Hence, the damages to the cells of that organ by alpha particles is substantially greater. Beta particles have greater penetrating power into the cells than alpha particles which causes the energy released by beta particles to be absorbed by a larger volume of tissue, bringing comparatively smaller damages to that organ.