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 making long-distance calls at night or on weekends, at a time which many wireless companies provide unlimited airtime for a small monthly fee
a) same as above
b) phones, instead using mobile phones to make long-distance calls during the night of weekends, during which
c) phones by using their mobile phones to make long-distance calls at night or on weekends, when
d) phones using mobile phones for making long-distance calls during the night or weekends, when
e) phones when using their mobile phones to make long-distance calls at night or on weekends, a time which.

My question is not about the answer to the underlined part, but rather about the structure: as the cost of wireless....., many people are finding that..

This structure must be correct but it seems to me that the first part of the sentence (as the cost of wireless service has steadily dropped over the last year) modifies (many people) which wouldn't make sense. I thought a more appropriate way would be (and again I'm not correcting GMAT official guide here, I'm just trying to find out what I'm doing wrong): as the cost of wireless service has steadily dropped over the last year and as mobile phones have become increasingly common, home phone charges are easily avoided.......(here the charges are modified by the first part of the sentence which makes more sense).
Would you please help me understand what it is I am assuming wrong? is my understanding of the structure correct? is the first part a modifier? if yes, am I right to assume that it should modify phone charges?
thanks in advance