Answer:
B - Failure to follow the rules of friendship.
Explanation:
A common cause of friendship failure is, as the exercise provides, the failure to follow the rules of friendship. A friendship is a relationship, a bond between people, in which both individuals should be equal and have no means outside the love and care for eachother. Even though every relation has different "rules", spoken or not, the failure to follow certain standars is probably the most common cause of failure. It's not uncommon that, when one breaks one of this rules, the other person feels betrayed and may even be able to forgive the "transgression" but not be able to recover the trust that has been lost.
