C: We have been here from 7 o'clock until now.
I said C is definitely wrong because I thought 'since' was used in the above sentence. That's why I replied "When 'since' is used 'until now' should not be used" and gave the following sentences.
We have been here since 7 o'clock.
I have been a teacher since 1990.
C: We have been here from 7 o'clock until now. (As 'since' is not used here, the sentence is not wrong, as CJ has pointed out. However, as it is a long-winded way, the 'since' version should, in my opinion, be used instead.)