An explosion blows a rock into three parts Two parts go off at right angles to each other. These two are, 1 kg first part moving with a velocity of $12ms^{-1}$ and 2 kg second part moving with a velocity of $8ms^{-1}$ If the thirds part files off wi a velocity of $4ms^{-1}$, its mass would be :

Question

( A ) 3 kg
( B ) 17 kg
( C ) 7 kg
( D ) 5 kg