When Joseph sees a frail man limping around the corner he owns, he will likely help if the costs of helping are small and Joseph will feel better about himself helping.
Helping someone in need corresponds to a form of empathy and altruism, which is based on the moral emotions we feel when we see a person in a situation that we have the ability to help.
According to the theory of social exchange, altruism only exists if the benefits to the helper outweigh the cost, so supported by this theory, the letter b is correct, and Joseph will help the man if the costs of helping are small.
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