I have hard time understating Contingent Valuation, please help, thanks

Problems with Contingent Valuation is that people trying to value X, such as saving whales, O O O O O is that the amounts vary significantly depending upon whether the survey asked about just whales, or whales and seals, for example. Is that the amount tends not to vary proportionately with the number saved Is that people know exactly how to value X, like another whale saved is that it is very hard to estimate the social discount rate A and B