21、在完全發展階段的圓管中，其速度分佈為，其中 dp / dx、μ 分別為壓力梯度以及黏滯係數。下列敘述何者為真？

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。