If $a^{2}=9$, then $a^{4}$ equals
(A) 27
(B) 81
(C) 243
(D) 729
(E) 2187
Since $a^{2}=9$, then $a^{4}=\left(a^{2}\right)^{2}=9^{2}=81$.

Alternatively, we note that since $a^{2}=9$, then $a= \pm 3$. If $a=3$, then $a^{4}=3^{4}=81$ and if $a=-3$, then $a^{4}=(-3)^{4}=3^{4}=81$.

ANswer: (B)