Finding the efficient level of abatement

Given a table of info with level of abatement in percent, total cost in dollars, and total benefit in dollars how do you identitfy what the most efficient level of abatement is by general economic criteria?

with this data how do you perform the cost benefit test to tell if at a given percentage level abatement is effiecient?

if someone argues for abatement at 100% but that is not efficient - do you respond that it is not cost effective to society at that level? what is a better responce? please describe