II.B.10.     A rule of thumb often used says a radioisotope may be treated as "non-radioactive" at the end of 10 half-lives.     What fraction of the radioisotope actually remains at this time?

    a.     1 - (1/2)¹⁰

    b.     1 - e^-0.693 x 10

    c.     (1/2)¹⁰