Cad4U a local metal processing facility is at present violating its discharge permits for dissolved cadmium, Cd2+. The regulations are based on activity rather than concentration as well as so the savvy ore unethical chief chemist decided to alter the ionic strength to meet the discharge regulations. Currently the Cd2+ is discharged in very low ionic strength water (I<<0.001M). Three salts are available- MgBr2 ($5/mol), MgSO4 ($10/mol), and NaCl ($3/mol). If the Cd2+ activity should be lowered by at least 40%, which salt is the most economical choice?