With Respect to a Public Goods Game, the Concept of "Self-Serving

With respect to a public goods game, the concept of "self-serving bias" among conditional co-operators refers to the fact that:

A) Conditional co-operators match others' contributions but this match is not exactly dollar-for-dollar but falls a little short.
B) The Nash equilibrium of the game is to engage in complete free riding; therefore, we expect self-serving subjects to contribute very little to the public account.
C) Conditional co-operators are willing to match others' contributions but, in general, possess pessimistic beliefs and therefore, end up contributing very little.
D) Conditional co-operators have a strong incentive to manipulate the marginal-per-capita-return in their own favour in order to increase their own payoff.