Either food scarcity or excessive hunting can threaten a population of animals. If the group faces food scarcity, individuals in the group will reach reproductive maturity later than otherwise. If the group faces excessive hunting, individuals that reach reproductive maturity earlier will come to predominate. Ttherefore, it should be possible to determine whether prehistoric mastodons became extinct because of food scarcity or human hunting, since there are fossilized mastodon remains from both before and after mastodon populations declined, and __. Which of the following most logically completes the reasoning? ['it is not known when humans first began hunting mastodons', 'it can be accurately estimated from fossilized remains when mastodons became extinct', 'the average age at which mastodons from a given period reached reproductive maturity can be established from their fossilized remains', 'climate changes may have gradually reduced the food available to mastodons']