How Do Internet Polling Companies Like Knowledge Networks Ensure Participation

How do Internet polling companies like Knowledge Networks ensure participation in its Web-based surveys?

A) Respondents are paid a small sum every time they participate.
B) Respondents are selected from the cell-phone-only segment of the population.-Consider This: The polling company does originally contact potential participants by phone but they do not restrict this to cell-phones.
C) Respondents are randomly sampled.
D) Respondents are provided a free computer.