Please choose the correct answer to the question: A scientist finds the bones of a dinosaur. What could help the scientist determine the approximate age of the dinosaur bones?

Answer1: the birds living in the area of the bones Answer2: the weather conditions in the area of the bones Answer3: the kinds of trees living in the area of the bones Answer4: the index fossils in the area of the bones

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