Cad4U, a local metal processing facility, is currently violating its discharge permits for dissolved cadmium, Cd2. The regulations are based on activity rather than concentration, and so the savvy (or unethical) chief chemist decides to alter the ionic strength to meet the discharge regulations. Currently the Cd2 is discharged in very low ionic strength water (I << 0.001 M). Three salts are available: MgBr2 ($5/mol), MgSO4 ($10/mol), and NaCl ($3/mol). If the Cd2 activity must be lowered by at least 40%, which salt is the most economical choice? Use the CEE extended Debye-Hückel equation and assume [Cd2+] is constant at 0.01 M. (The value for effective diameter, a, is 5 x 10-8 cm).