Momentum is conserved if and only if sum of all forces which are exserted on system equals zero. In our situation there are only internal forces, so by Newton's third law their vector sum is 0.
So [tex]mv=(m+M)v' \Leftrightarrow v'=\frac{mv}{m+M}[/tex].
Kinetic energy of system at first: [tex]\frac{mv^2}{2}=784\;\textbf{J}[/tex]. After: [tex]\frac{(m+M)\frac{m^2v^2}{(m+M)^2} }{2}=\frac{m^2v^2}{2(m+M)}\approx 1,96\; \textbf{J}[/tex]. The secret is that other energy is in work of deformation forces (they in turn heat a bullet and a block).
Answer is A)
