If $N_0$ is the original mass of the substance of half-life period $t_{1/2} = 5\; years$, then the amount of substance left after 15 years is

Question

( A ) $\frac{N_0}{2}$
( B ) $\frac{N_0}{8}$
( C ) $\frac{N_0}{4}$
( D ) $\frac{N_0}{16}$