Please choose the correct answer to the question: Biologists have discovered that the beluga whales of Canada's St. Lawrence River have the highest rates of cancer among animals. What is most likely the reason for this high incidence of cancer in these whales?

Answer1: chemical and pollutant runoff into the ocean Answer2: average temperatures increased in the region Answer3: depletion of natural prey from the ecosystem Answer4: increased levels of carbon dioxide in the atmosphere

Answer format: answer1/answer2/answer3/answer4
Answer: answer1