The answer is most likely C
A dependent clause adds something to a sentence but cannot be a standalone sentence, hence "dependent." In this sentence, "when I had... phone" is a dependent clause.
Have you ever watched a dull movie? Have you been bored watching the teacher present slides? Well imagine the teacher presenting every single slide, and each slide has over 1000+ words. Yeah, you would be bored. This is why using the main idea in a presentation is needed; it prevents long slides that could take hours to finish, but it shortens the slide to a point where it can be easily understood without unnecessary detail.
However, unlike (point A), (point B) actually
Or some other buzz words are:
Inversely
Dissimilarly
Otherwise
Unequally
Contrarily
