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答案：D
解析：
Both a-b = 6 and b -3 = 6. The equation a-b = 6 cannot be solved for a because there are two unknowns. The value of b can be found by solving the equation - 3 = 6 for b. It follows that b must equal 9. The value of b can now be substituted into the equation a-b = 6, giving a-9 = 6. Therefore, a must equal 15.
