how can respondents' income affect research outcomes in studies of contingent evaluation? willingness to pay studies are affected because people with high incomes have a higher willingness to pay for anything. travel costs studies are affected because high income who spend more money visiting an environmentally beautiful spot do not necessarily value it less than a poorer visitor with less money to spend. hedonic regression studies are affected because people with higher incomes can spend more money to be far away from polluted site than a less prosperous person. all of the above none of the above