In a study of 100 New York City restaurants, a control group had customers pay their bill in cash with the bill presented on a tray that bears no credit card logo. The experimental group had customers pay their bills in cash with the bill presented on a tray that does bear a credit card logo. Customers in the experimental group left larger tips on their bills. Consumer psychologists believe that the presence of the credit-card logo makes customers, many of whom have credit cards, willing to spend more because it reminds them that their spending power exceeds the cash they have immediately available.
Which of the following, if true, most strongly supports the psychologists' hypothesis?