Hi friends, The problem is from Newton's Laws. The problem is as follows: https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-ash4/1006364_1417581381802301_666606151_n.jpg IInd law states, Force, F = dP/dt => F = d(mv)/ dt Out come, If m is constant, v is variable, F = m.[d(v)/dt] => F = m.a If v is constant, m is variable, F = v.[d(m)/dt] => F = v.[rate of change of mass] If both m and v are variable, F = m.[d(v)/dt] + v.[d(m)/dt] Hence the answer of the question should be Option (B). But the book states that answer is option (C) is correct. How is it so. Please friends help me in solving this issue. Thank you all in advance. I would appreciate the help.