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 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

(D)

I don't even know the rule being tested here =(