Explanation

(c)
Volume of balloon $= \frac{4}{3}\pi r^{3}$
$= \frac{4}{3} \times \frac{22}{7} \times \left( \frac{21}{2} \right)^{3} = 4851\ mL$
Volume of the cylinder containing gas =2.82 L = 2820 mL
Volume at STP $= \ V_{1} = 2820 \times \frac{273}{300} \times 20 = 51324$ mL
Volume of the gas that will remain in the cylinder after filling balloons is equal to the volume of cylinder, $ie,$ 2820 mL
Available hydrogen for filling
$= 51324 - 2820$
$= 48504\ mL$
Number of balloons $= \frac{48504}{4851} \approx 10$