Comparative income statements for Tudor Retailing Company for the last two months are presented below:

	September	October
Sales in units	5,000	7,000
Sales revenue	$100,000	$140,000
Cost of goods sold	40,000	56,000
Gross margin	60,000	84,000
Selling and administrative expenses:
Shipping expense	7,500	10,500
Clerical expense	10,000	12,000
Maintenance expense	17,000	17,000
Total selling and administrative expense	34,500	39,500
Net operating income	$25,500	$44,500

Which of the following classifications best describes the behavior of shipping expense?

Mixed

Variable

Fixed

none of these