Why selling costs are more important under oligopoly than monopolistic competition as it has large no. of firms than oligopoly?

Dear Student
In oligopoly market, the competition among the few firms is very intense. Each firm continuously monitors the actions of the other firms in the market and each of them wants to have an edge over the other firms. In order to convince the customers to buy their products, a large sum of selling costs are incurred by the firms. 
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