【单选题】Which statement about protein-ligand binding is CORRECT...

【单选题】Which statement about protein-ligand binding is CORRECT?The K a is equal to the concentration of ligand when all of the binding sites are occupied. The K a is independent of such conditions as salt concentration and pH. The larger the K a (association constant), the weaker the affinity. The larger the K a , the faster is the binding. The larger the K a , the smaller the K d (dissociation constant).