The escape velocity of a body on a planet $A$ is $12 \mathrm {~km} \mathrm {~s} ^ { - 1 }$. The escape velocity of the body on another planet $B$, whose density is four times and radius is half of the planet $A$, is\\
(1) $12 \mathrm {~km} \mathrm {~s} ^ { - 1 }$\\
(2) $24 \mathrm {~km} \mathrm {~s} ^ { - 1 }$\\
(3) $36 \mathrm {~km} \mathrm {~s} ^ { - 1 }$\\
(4) $6 \mathrm {~km} \mathrm {~s} ^ { - 1 }$