Manero Company included the following information in its annual report:

	2019				2018				2017
Sales	$	178,400			$	162,500			$	155,500
Cost of goods sold		115,000				102,500				100,000
Operating expenses		50,000				50,000				45,000
Operating income		13,400				10,000				10,500

In comparison to year 2018, the increase in operating income of 2019 was primarily caused by the effect of margin increase of (ignore taxes):

a $978.

b $1,194

c $2,422

d $3,400.