A spherical weather balloon is filled with hydrogen until its radius is $2\mathrm{.}30$ m$.$ Its total mass including the instruments it carries is $21\mathrm{.}0$ kg$.$

$($a$)$ Find the buoyant force acting on the balloon, assuming the density of air is $1\mathrm{.}29$ kg$/$m$3.$

kN

$($b$)$ What is the net force acting on the balloon and its instruments after the balloon is released from the ground?

kN

$($c$)$ Why does the radius of the balloon tend to increase as it rises to higher altitude?

Because the external pressure $---$Select$---$increases$$decreases with increasing altitude.