Each month the U. S. Census Bureau's American Community Survey mails survey forms to 250,000 households asking questions about demographic, social, economic, and housing characteristics such as mortgage and utility costs. Telephone calls are made to households that don't return the form. In one month, responses were obtained from 240,000 of the households contacted. A source of bias in this survey is

a. voluntary response
b. the fact that the survey was not double-blind
c. nonresponse
d. only U. S. households were contacted