To Which of the Following Two-Object Systems Can We Apply $C$

Short Answer

To which of the following two-object systems can we apply conservation of momentum, and why? In each case, answer "A" if we can apply conservation of momentum because the system floats in space, "F" if it's because the system is functionally isolated, " $C$ " if it's because the system undergoes a collision, and " $\mathrm{D}$ " if momentum is not conserved at all because the system is not isolated.
-A cannonball drops to the earth and lands with a thud, burying itself in the ground.