请教老师们：These things did not happen to these people because they were lazy or stupid. They happened because of addiction. New college students，even the brightest and most successful ones，are most at risk from these effects because the Internet is important for their studies and because they are just entering （a）world where their online habits are no longer monitored by concerned parents. 中为什么world前用a而不是the呢？ 我认为the:(1)从上文可以看出world指大学生活呀 (2)后面不是有定语从句吗？