Allele Z Codes for a Cooperative Behavior Among Half Siblings

Allele Z codes for a cooperative behavior among half siblings (r = 0.25) . Recipients of this behavior receive a benefit of four fitness units and the behavior costs the donor two fitness units. Using Hamilton's rule (rb - c > 0) , would you expect allele Z to spread in the population?

A) Yes; allele Z would increase in frequency within the population.
B) No; allele Z would decrease in frequency within the population.
C) Allele Z would neither increase nor decrease in frequency.
D) There is not enough information to determine if the allele will spread.