In oblique projectile motion, the power of the gravitational force 
A) Is negative for first half, zero at top most point and positive for rest half 
B) Varies linearly with time.
 

(A) $\mathrm{P}=\overline{{\mathrm{F}}_{\mathrm{g}}}\overline{. \mathrm{V}}={\mathrm{F}}_{\mathrm{g}}\mathrm{Vcos}\mathrm{\theta }$
For the first half, $\mathrm{\theta }$ is greater than hence p is negative 
At the top most point $\mathrm{\theta }$ = 90° , hence P=0
For the recand half, $\mathrm{\theta }<{90}^0$hence P is positive
(B) $\begin{array}{l}\mathrm{P}=\overline{{\mathrm{F}}_{\mathrm{g}}}\overline{\mathrm{V}}\\ =-\mathrm{mg}\widehat{\mathrm{j}}\cdot \{\mathrm{ucos}\widehat{\mathrm{i}}+(\mathrm{usin}\mathrm{\theta }-\mathrm{gt}\left)\widehat{\mathrm{j}}\right\}\\ =\mathrm{mg}\{\mathrm{gt}-\mathrm{usin}\mathrm{\theta }\}\end{array}$