问一道搞不懂的语法

lllmmm

***************** entirely to shows
produced specifically for children.
(A) Although at one time children's programming had been limited to a few hours a week, now,
however, there are at least four cable networks that dedicate themselves
(B) With children's programming having been limited at one time to a few hours a week, now at
least four cable networks have dedicated themselves
(C) Once limited to a few hours a week, there are now at least four cable networks with children's
programming that dedicate themselves
(D) Children's programming was once limited to a few hours a week, but now there are at least four
cable networks dedicated
(E) While children's programming had once been limited to a few hours a week, at least four cable
networks are now dedicated

这道题E为什么不对请问   ？

pygmysog

had been是过去完成时，只有在有明显的过去时动词出现时才会用到过去完成时

hldada

恩，同意楼上，时态是主要错误，但我也不喜欢这里用while 总觉得有点怪怪的，呵呵

爱吃巧克力

答案是D么？
E中，had done 错误，过去完成时的使用一定要有明确的提示证明发生在过去的过去。而这里连一个过去的动作都找不到，何来过去的过去
另外，值得一提的是while在gmat中更多地表示同时发生，表示对比的较少

lllmmm

答案是D