Statement I : For two bodies, the sum of the mutual forces exerted between them is zero from Newton's third law.Statement II : The sum of work done by the two forces must always cancel.

For two bodies, the sum of the mutual forces exerted between them is zero from Newton's third law.i.e. F12 + F21 = 0But the sum of the work done by the two forces need not always cancel.i.e. W12+ W21 = 0However, it may sometimes be true.