Surveys Sent by Mail Often Have Low Response Rates

Multiple Choice

Surveys sent by mail often have low response rates. Which technique is likely to be more effective in increasing the number of responses:

A) Including a "gift" check for $5 with the survey.
B) Offering to send respondents $5 after they have returned the survey.
C) Both a and b will be equally effective
D) Neither a nor b will be effective