Mr.D doesn't die, because the radius of the boulder is 1 square, and the length that the boulder goes is 5 squares, wich means that when the boulder reachs D, the missing part will be facing the ground, thus saving mr.D. As for Mr. C, the rolling boulder wheights less than the one thats on the other end, so he doesn't die too. I guess i'm bad at english, sorry about that