A stationary body of mass $m$ explodes into 3 parts with mass ratio of $1: 3: 3$. The two fragments with equal mass move at right angles to each other with velocity of $15 \mathrm{~m} \mathrm{~s}^{-1}$. The velocity of the third fragment is $\left(\mathrm{m} \mathrm{s}^{-1}\right)$
(a) $\sqrt{2}$
(b) 5
(c) $20 \sqrt{2}$
(d) $45 \sqrt{2}$