複選題51、焠火硬化的鋼材經回火後，可以： (A)降低內應力 (B)消除偏析(segregation) (C)增加硬度 (D)改善韌性。

10、 In Figure 10, a horizontal force of 500 N acts on a block of weight 1.5 kN placed on an inclined plane. The coefficients of friction between the block and the plane are μs =0.25 andμk = 0.20 . What is the actual frictional force? (A) 200N (B) 500 N (C) 300 N (D) 450 N。