Answer : In some ways, it can be all. However, B tends to be a very prominent issue for amateur writers.
For those who are only just beginning to craft dialogue, their characters do not have any unique way of speech that makes them stand out. If they were to cover the name of who is speaking, amateur writers commonly cannot tell to whom the dialogue belongs. Each character is equally likely to make a sarcastic statement as they are a heartfelt one, and there is little distinction between Person A and Person B.