Respuesta :
Answer: (3) historically, vice presidents and their families lived in their own homes, but the cost of securing these private residences grew substantially over the years.
Explanation:
Although the fourth sentence explicitly points to the naval observatory as the place chosen for the vice president to live in, it is in the third sentence that the reason behind that decision is stated: it was too costly to make their private homes secure enough for them, so they had to be moved to the naval observatory.