How Does Evolutionary Theory Explain Helping Among Nonrelatives

How does evolutionary theory explain helping among nonrelatives?

A) Helping among nonrelatives occurs in human groups but not in animal groups, and is not related to the survival of one's genes.
B) Helping among nonrelatives rarely occurs and is often an act of pure altruism.
C) Helping among nonrelatives occurs in order to ensure survival of diverse genes.
D) Helping is often mutual and cooperative, so the helpers benefit by being helped in return.