The average speed of a car running at the rate of 15 km/hr during the first 30 km at 20 km/hr during the second 30 km and at 25 km/hr during the third 30 km is

Question

$\begin {array} {1 1} (A)\;19.15\: km/hr & \quad (B)\;20\: km/hr\\ (C)\;19.65\: km/hr & \quad (D)\;19.75\: km/hr \end {array}$