Read the passage carefully and choose the correct answer.

THE TRAM SYSTEM IN HA NOI: THEN AND NOW

            Hanoi had its first tramways in 1900. The tram system was a major means of transport in the city for nine decades, and thus the image of the tram and its clanging sounds have gone deep into the hearts and minds of Hanoians. Since it was convenient and cheap to get around the city and to the suburban areas by tram, the system was very popular. However, due to the population boom and the need for a wider road system, the last rail track was removed.

            Now, after more than 20 years, the population of Hanoi has risen from about two million people to more than six million people; therefore, the number of vehicles on the roads has increased dramatically. To meet the increasing travel demands, Hanoi is launching its first sky-train system, connecting Cat Linh and Ha Dong. The system is expected to begin operations by 2016, with initial four-compartment sky-trains. Furthermore, a new rail system project including around eight kilometers of sky-train rail and four kilometres of subway rail, connecting Hanoi Central Station and Nhon, has also been under construction and is expected to be completed in a few years.

The population of Hanoi has increased remarkably within ______.