verbal Q8 新问题~

sunnyybao

As the cost of wireless service has steadily dropped over the last year and as mobile phones have become increasingly common, many people are finding that they can avoid toll charges on their home （phones, using their mobile phones for make long-distance calls at night or on weekends, at a time which） many wireless compaies provide unlimited airtime for a small monthly fee.
A. phones, using their mobile phones for make long-distance calls at night or on weekends, at a time which
B. phones by using their mobile phones to make long-distance calls at night or on weekends, when

答案是B，很明显B好，可是OG中说  “The phrase at a time which is confuding because it seems to suggest a time other than at night or on weekends."
不是很理解，麻烦大家帮忙解释下吧~

sunnyybao

up~up~