* A question about S+V+O+When+ PRESENT PARTICIPLE

[wangyinwei_2005]

Hi, instructors:
I want to ask a about the using of "˜S+V+O+ when+ present participle’
As I saw on manhattan forum before, I have already known that if you say "when + PAST PARTICIPLE", then this automatically applies to the SUBJECT of the clause to which it's attached.
an example from Ron:
iron accumulates rust when submerged in water
then it's the iron, not the rust, that's submerged in water
but what about when+ PRESENT PARTICIPLE，
here is a sentence:
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 when using their mobile phones to make long-distance calls at night or on weekends, a time which many wireless companies provide unlimited airtime for a small monthly fee.
this is a wrong answer of a sc question; so here what's the subject of "when using..."? is it 'they' i remarked with red?
Thank you in advance.

[RonPurewal]

you have correctly identified the agent of "using...". so, that part of the sentence is fine. (remember, just because a choice is wrong, that doesn't mean everything in that choice is wrong!)

please follow the forum rules, though:
* post the question together with all after choices
* cite the original source of the problem
please do these two things, or else we will have to delete the post. thank you.