Overexposure to certain wavelengths of strong sunlight is the main cause of melanoma, a virulent form of skin cancer. For this reason, doctors now urge everyone to put adequate sunblock on skin exposed to strong sunlight. Adequate sunblock, according to doctors, is any preparation that prevents sunburn even if the person is exposed to strong sunlight for a significant length of time. Which one of the following, if true, most weakens the recommendation that people wear adequate sunblock? ['Sunburns appear immediately after exposure to the sun but melanoma appears years after repeated exposures.', 'There is no evidence that there are wavelengths of sunlight that lead to both sunburn and melanoma.', 'Toxins contained in certain chemical compounds also cause melanoma.', 'There are people who have allergic reactions to certain chemicals found in many sunblocks.']