Return Missile vs Explosive Fireball [DF]

In a rather epic turn of events last night, our campaign's wizard used Return Missile to block an orc shaman's a 9d explosive fireball aimed at the ground where the cleric was standing over the body of a fallen party member. The return missile succeeded, and while the offending caster succeeded with a dodge and drop, he still took 17 damage on 3d6 , and burned for quite a while. In retrospect, I have seen that spells CAN target the ground, so the blocking spell was used properly. (As well as being very entertaining!)



But I wonder, if you were DM, and there were essentially 3 targets in the hex, Cleric, downed party member, and ground . Would you make the wizard roll something to determine the target, or just let him guess and pick one, or just tell him the correct target? If he targets the ground but the missile is not an explosive fireball, but a regular one, the block would have the wrong target, and fail. (insert evil dm chuckle here)

Return Missile vs Explosive Fireball [DF]