An explosion blow a rock into three parts. Two parts go off at right angles to each other.

An explosion blow 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 12 ms-1 and 2 kg second part moving with a velocity 8 ms-1. If the third part flies off with a velocity of 4 ms-1, its mass would be:
(a) 7 kg
(b) 17 kg
(c) 3 kg
(d) 5 kg
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