A solid sphere of radius, $r$, and made of material with density, $\rho_s$, is moving through the atmosphere (constant pressure, $p$) with a velocity, $v$. The net force ONLY due to atmospheric pressure $(F_p)$ acting on the sphere at any time, $t$, is

1. $\pi r^2 p$
2. $4 \pi r^2 p$
3. $\dfrac{4}{3} \pi r^3 \rho_s \dfrac{dv}{dt}$
4. zero