单选题
A man drove his automobile d1 kilometers at the rate of r1 kilometers per hour and an additional d2 kilometers at the rate of r2 kilometers per hour. In terms of d1, d2, r1, r2, what was his average speed, in kilometers per hour, for the entire trip?
【正确答案】
A
【答案解析】[解析] 总的平均速度等于总路程除以总时间，这个人开车所行驶的总路程为d1+d2，开车所用的总时间为[*]，所以其总的平均速度为：[*]