Please explain your choices? I wanted to double check to see if I understood the questions correctly

(d) When sales increase by 21%, which of the following should also increase by 21% 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.) V Variable cost ? Fixed cost ? Gross margin V Contribution margin ? Net operating income (e) When sales increase by 21%, which of the following should increase by more than 21% 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.) 2 Variable cost ? Fixed cost Gross margin ? Contribution margin Net operating income