$19)$ Free fall: On the earth, when an astronaut throws a $0\mathrm{.}250-$kg stone vertically upward, it returns to his hand a time T later. On planet X he finds that, under the same circumstances, the stone returns to his hand in $2$T$.$ In both cases, he throws the stone with the same initial velocity and it feels negligible air resistance. The acceleration due to gravity on planet X $($in terms of g$)$ is A$)$ g $2.$ B$)$ g$/2.$ C$)$ g$/4.$ D$)$ g$/2.$ E$)2$g