A body of mass $m$, accelerates uniformly from rest to $v _ { 1 }$ in time $t _ { 1 }$. The instantaneous power delivered to the body as a function of time $t$ is\\
(1) $\frac { m v _ { 1 } t } { t _ { 1 } }$\\
(2) $\frac { m v _ { 1 } ^ { 2 } t } { t _ { 1 } ^ { 2 } }$\\
(3) $\frac { m v _ { 1 } t ^ { 2 } } { t _ { 1 } }$\\
(4) $\frac { m v _ { 1 } ^ { 2 } t } { t _ { 1 } }$