Based on the information provided in the article, the best answer is:
D. They both enabled teenagers to develop social connections outside the home.
The development of the car and compulsory education both contributed to teenagers' ability to interact and socialize outside of their immediate family and home environment. The car provided mobility and independence, allowing teenagers to gather with friends and engage in social activities outside of the home. Similarly, compulsory education increased the number of teenagers in high schools, providing them with opportunities to interact with their peers, form social connections, and participate in shared experiences within an educational setting.