I believe the best answer is both A and B, that is there is no known way to shorten a half life, radioactivity is limited by the natural decay-time to stable isotopes, and again all half-lives are long, there is no known way to measure half-lives are long, there is no known way to measure half-lives with any accuracy. Half life is the time taken by a radioactive material to undergo decay to reach half of its original mass.
