Toyota's average cost of producing 3 cars is $\textbackslash$30,000$ and its average cost of producing 4 cars is $\textbackslash$25,000$. Find Toyota's total cost of producing 3 and 4 cars. What was the marginal cost of the 4th car? 
选项为:A:$5000  B:$90000  C:$10000  D:$55000  请回答其中一个选项.