Montclair Company earns an average contribution margin ratio of 40% on its sales. The local store manager estimates that he can increase monthly sales volume by $45,000 by spending an additional $7,000 per month for direct mail advertising. Compute the monthly increase in operating income if the manager's estimate about the increased sales volume is accurate.(a) $11,000.(b) $23,000.(c) $16,000.(d) $18,000.