A sample of cells with a total receptor concentration of 50 mM was incubated with a ligand concentration of 100 mM. Following the incubation period, the concentration of unbound receptors was determined to be 15 mM. What is the Kd for the receptor-ligand interaction?
A) 28 mM
B) 43 mM
C) 143 mM
D) 217 mM
E) 333 mM