Alpha-emitting substances, such as radon gas, can be a serious health hazard only if A. your eyes are open. B. their radiation strikes the skin. C. exposure to them is external. D. none of the above

Alpha emitters have very poor penetration ability. So it takes very little for them to be blocked. But what happens if you get them inside you? They simply radiate over and over the immediate area of the radioactive particle. If they were strong emitters, the radioactivity would simply pass out of your body, doing far less damage to tissue.

So if you haven't figured out yet, the answer is (A).