Four vehicles are traveling on a 4-kilometer circular track. The four vehicles are traveling at constant speeds of 70 km/h, 63 km/hr, 75 km/hr, and 84 km/hr, respectively. Assume you are an observer standing at a point on the track for a period of 30 minutes and are recording the instantaneous speed of each vehicle as it crosses your point.

Which option below represents most accurately the time-mean speed for these vehicles during this observation period?