A radioactive substance has a half-life of four months. Three-fourth of the substance would decay in

. . . . . . . . invented Atom Bomb.

A. J Robert Oppenheimer

B. John Bau a Ning

C. Samuel Cohan

D. Samuel Colt

Positive rays were discovered by

A. Thomson

B. Goldstein

C. W-Crookes

D. Rutherford

Cathode rays are

A. positive rays

B. neutral rays

C. helium rays

D. electron waves

X-ray can be produced

A. when low speed electrons is ejected

B. when high speed electron is obstructed by a metal surface

C. when anode-ray obstructed by a solid object

D. when γ-ray is obstructed by a solid object