Problem

A radioactive sample

S_1

having an activity of

5 \mu\text{Ci}

has twice the number of nuclei as another sample

S_2

which has an activity of

10 \mu\text{Ci}

. The half-lives of

S_1

and

S_2

can be (JEE 2008) a. 20 yr and 5 yr, respectively b. 20 yr and 10 yr, respectively c. 10 yr each d. 5 yr each