Ligand A binds to protein P with a dissociation constant, ( K_d = 0.20 , muM ). What concentration of A is required in order for 25% of P to be bound?
a) ( 0.05 , muM )
b) ( 0.10 , muM )
c) ( 0.20 , muM )
d) ( 0.40 , muM )