B. When the car is driving on the expressway, the child sitting in Xiao Zhang's car feels that the car next to it has not moved, indicating that the two cars are relatively static, so it is based on itself, so B is correct;
C, where 8: 00 and 9: 50 refer to time, the interval between them is time and time scalar, so C is wrong;
D, according to the data in the question, you can find the average speed of Xiao Zhang's whole journey, but you can't find the average speed, so D is wrong;
Therefore, choose: B.