如图所示，长方形斜面倾角为37°，其长为0.8m，宽为0.6m，一重为25N的木块原先在斜面上部，它与斜面间的动摩擦因数μ=0.6，现对它施以平行斜面的恒力F，使其沿对角线AC方向匀速下滑，则木块所受摩擦力大小是（　　）

A．20 N

B．15N

C．12N

D．9 N

Brian is Paul’s father. In January this year, Paul orally agreed with Brian to decorate Brian’s apartment at cost. Subsequently, they signed a written agreement, which reflected all the terms that they had agreed. In April, Paul completed the decoration. However, Brian has refused to pay to Paul the cost.Required:In relation to contract law, advise Paul as to whether he can commence legal action against Brian to recover the cost. (10 marks)