With Regards to the Public Goods Game, the Phenomenon of "Conditional

With regards to the public goods game, the phenomenon of "conditional cooperation" refers to the finding that:

A) Participants' contributions to the public account are strongly positively correlated with their prior beliefs regarding the expected contributions of their peers.
B) Participants' contributions to the public account are strongly negatively correlated with their prior beliefs regarding the expected contributions of their peers.
C) Participants' contributions to the public account are "hump-shaped"; increasing in the expected contribution of other group members up to a point but decreasing in the expected contribution of other group members thereafter.
D) Some participants always contribute their entire endowment to the public good regardless of what others do since this is commensurate with the social optimum.