题目

（5分）如图，放在倾斜木板上的物块，当木板倾角为30°时所受摩擦力和45°时所受摩擦力的大小相同，则物块和木板间的滑动摩擦系数为＿＿＿＿＿＿＿。 答案：— Did he do this job alone ?— No, he finished the job with the help of his parents.A. by himself B. with others C. quickly