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答案：D
解析：
Choice (D) is correct. Let r be the number of red marbles in the jar. The probability of selecting a red marble is equal to the number of red marbles divided by the total number of marbles. Since the probability of selecting a red marble is 2/3.There are 36 green marbles and r red marbles, so the previous equation becomes 2/3=r/r+36. This simplifies to 2(r+36)=3r, or 2r+72=3r. Therefore, the number of red marbles, r, is 72.
