Economists believe that an activity should be continued up to the point where
a. the marginal benefit from the activity is equal to the marginal cost.
b. the marginal benefit from the activity is less than the marginal cost.
c. the marginal benefit from the activity plus the marginal cost equals zero.
d. the marginal benefit from the activity is greater than the marginal cost.

Kirnon clinic uses client-visits as its measure of activity. during july, the clinic budgeted for 3,250 client-visits, but its actual level of activity was 3,160 client-visits. the clinic has provided the following data concerning the formulas to be used in its budgeting: fixed element per month variable element per client-visitrevenue - $ 39.10personnel expenses $ 35,100 $ 10.30medical supplies 1,100 7.10occupancy expenses 8,100 1.10administrative expenses 5,100 0.20total expenses $ 49,400 $ 18.70the activity variance for net operating income in july would be closest to: