Provided a ground rule does not supersede a rules book rule, it becomes in force when:

• A. Both coaches agree to the proposed ground rules.
• B. The umpires formulate the ground rules if the teams cannot agree.
• C. Only the umpires can create the ground rules for the game.
• D. Both A and B.

Answer: (D)

Ground rules are local additions that must not override the official rules. They come into force either when both coaches agree to the proposed ground rules, or, if the teams cannot agree, the umpires formulate them for the game. This ensures there is mutual consent or official oversight, and nothing local can contradict the rules book.

So the most complete understanding is that ground rules become in force through either agreement of both coaches or, in the absence of agreement, the umpires establishing them.