A taxi driver make a trip from the plains to ooty which are 340 km apart at an average speed of 60 km/hr. In the return trip, he covers the speed distance at an average of 30 km/hr. The average speed of the taxi over the entire distance of 680 km is:

(A) 30 km/hr
(B) 40 km/hr
(C) 50 km/hr
(D) 60 km/hr