the internet is , I do not think it is a good idea to spend too much time on it.
A. When ; helpful
B. As ; great help
C. While ; of great help
D. Now ; of no help
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A. When ; helpful
B. As ; great help
(答案→)C. While ; of great help
D. Now ; of no help
解析：考查连词。句中while作“虽然”之意；be of great help = very helpful。句意：虽然网络是很有帮助的，但我认为在它上面花太多的时间不是一个好主意。