When amphibians first appeared on Earth millions of years ago, the radiation of ultraviolet rays penetrating the atmosphere was much greater than it is now. Therefore, the current dramatic decrease in the number of amphibians is not the result of the recent increase in ultraviolet rays penetrating the earth's atmosphere. Which of the following is the assumption on which the above discussion is based? ['Eggs of modern amphibians are no more vulnerable to UV radiation than eggs of early amphibians.', 'Modern amphibians are unlikely to be sheltered from UV rays in habitats that lived like early amphibians.', 'Modern amphibians cannot adapt to changing levels of radiation as easily as early amphibians.', 'The skin of amphibians is generally more sensitive to UV light than the skin of other animals.']