A nutritional consulting company is trying to find what percentage of the population of a town is overweight. The marketing department of the company contacts by telephone 600 people from a list of the town's telephone directory. All 600 people give answers to the survey. Which of the following is the most significant source of bias in this survey?

a. Size of sample.
b. Undercoverage.
c. Voluntary response bias
d. Nonresponse.
e. Response bias.