Professor Winer sent a mail questionnaire to 2,000 subjects. Winer told subjects that their responses would be anonymous. After the first mailing, 400 completed surveys were returned. For a second follow-up mailing, Winer should mail

a. 2,000 questionnaires.
b. 1,000 questionnaires.
c. 400 questionnaires.
d. 1,600 questionnaires.
e. 50 questionnaires.