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答案：C
解析：
To evaluate the expression 3(y-x), it is necessary to determine the values of x and y. Since the sum of the measures of the angles of any triangle is 180 degrees, and a right angle has measure 90 degrees, the value of x can be found by solving the equation x + x + 90 = 180. This yields x=45. The value of y can be found by solving the equation y + y + y = 180. This yields y=60. Finally, the values for x and y are substituted into the expression 3(y-x) to obtain 3(60-45) = 3(15) = 45.
