关于这个问题，有很多原因。一个主要原因是车辆增加的数量远快于道路的建设。另一个主要原因是私家车过多而公交车不够。
There are several reasons for this problem. One of the main reasons is that the number of vehicles is increasing much more rapidly than building of roads.
有一些主要原因。其中一个原因是……，另一个原因是……，而最经常被强调的原因是…… …
There are some major causes for… One cause is… Another cause is… And the most frequently stressed cause is…
一个大原因是……。另一个原因是……。
为什么会…？一个原因是…，另一个原因是…；或许其主要原因是…
Why did …? For one thing …, for another …perhaps the primary reason is …
为什么会…？一个原因是…，另一个原因是…；或许其主要原因是…
Why did …? For one thing …, for another …perhaps the primary reason is …
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