A According to Newton's third law, the tension between two cars is equal, so A is wrong.

B, after pulling, two cars pull at the same time, and start moving at the same time. So, B is wrong.

C, the tension of the two cars is equal. According to Newton's second law, the total mass is large and the acceleration is small. From x= 12at2, the displacement is small in the same time, and then it reaches the midpoint. That is, someone in the car reached the midpoint of the two cars. So, C is correct.

D, there is no need to weigh the quality, but item C can determine which car is occupied. So, d is wrong.

So choose C.