Pollster: When opinion researchers need a population sample that reflects the demographic characteristics of the national population, they choose their sample on the basis of national census data. Not everyone participates in the national census, despite its being mandatory. If, however, census participation became voluntary, as some have proposed, the participation rate would be much lower. So if census participation became voluntary, polls designed to discover the opinions of the national population would have less accurate results. Which one of the following is an assumption on which the pollster's argument depends? ['Among people who do not currently participate in the national census, few, if any, would agree to participate if participation were voluntary.', 'Using data from the national census is not the only way for opinion researchers to get a population sample that reflects the demographic characteristics of the national population.', 'The group of people who would participate in a voluntary national census would differ in its demographic characteristics from the group of people who would participate in a mandatory national census.', 'The people who refuse to participate in opinion polls comprise a group with approximately the same demographic characteristics as the group of people who do not currently participate in the national census.']