15.A proton is projected toward a fixed nucleus of charge +Ze with velocity v. Initially the two particles are very far apart. When the proton is a distance R from the nucleus its velocity has decreased to 1/2 v. How far from the nucleus will the proton be when its velocity has dropped to 1/4 v? (A) 1/16 R (B) 1/4 R (C) 1/2 R (D) 4/5 R (E) None of these.