Suppose Burger King has run a major advertising campaign in the hopes of increasing monthly sales. To investigate the effectiveness of this campaign, Burger King randomly selected seven restaurants and recorded the monthly sales before and after the advertising. The following data represents these sales figures in thousands of dollars.

Restaurant	1	2	3	4	5	6
After	$123	$122	$145	$156	$160	$134
Before	$107	$110	$143	$168	$145	$125

If Population 1 is defined as sales after the advertising campaign and Population 2 is defined as sales before the advertising campaign, the mean of the matched-pair differences is ________. 6.1 7.0 11.9 13.2