Three construction workers can throw bricks vertically up with speeds of $4 \mathrm{~m} \mathrm{~s}^{-1}, 6 \mathrm{~m} \mathrm{~s}^{-1}$ and $12 \mathrm{~m} \mathrm{~s}^{-1}$ respectively. Working together, they can stack them on a platform of maximum height (in $\mathrm{m}$ )
(a) 5
(b) 10
(c) 15
(d) 20