Given Two Best Friends, It Has Been Found That Such

Given two best friends, it has been found that such friends are likely to engage in antiphonal laughter which:

A) fills the empty gaps in conversation.
B) signals that one is tracking what the other is saying.
C) is contagious and sequential, such that the laughter of one is followed by the laughter of the other.
D) occurs in unison, such that there is an overlap in the friends' bouts of laughter.