(d) When sales increase by 4%, which of the following should also increase by 4% in a merchandising company? (You may select more than one answer. Single click the box with the question mark to produce a check mark for a correct answer and double click the box with the question mark to empty the box for a wrong answer. Any boxes left with a question mark will be automatically graded as incorrect.) Variable cost Fixed cost Gross margin Contribution margin Net operating income