What does the word flat mean in British English

A．British English and American English are the same.
B．They are so different that Americans can' t understand Englishmen when they are talking.
C．They have different spelling and vocabulary but the same pronunciation.
D．They have slight different spelling vocabulary and pronunciation, but the people can understand each other.

A．The materials became cheaper.
B．There was more competition.
C．More people used the phones.
D．The phones were connected across the sea.