When amphibians first appeared on Earth millions of years ago, the amount of ultraviolet radiation penetrating Earth' s atmosphere was much greater than it is today. Ttherefore, current dramatic decreases in amphibian populations cannot be the result of recent increases in ultraviolet radiation penetrating Earth' s atmosphere. Which of the following is an assumption on which the argument depends? ['The skin of amphibians is generally more sensitive to ultraviolet radiation than the skin of other animals is.', 'The eggs of modern amphibians are not significantly more vulnerable to ultraviolet radiation than the eggs of the first amphibians were.', 'Modern amphibians are not as likely as the first amphibians were to live in habitats that shield them from ultraviolet radiation.', 'Populations of modern amphibians are not able to adapt to changing levels of radiation as readily as populations of early amphibians were.']