Correct answer choice is:
Pollution.
Explanation:
Early human population was free from pollution and lived a very healthy life. That's why they have long ages and have the potential to do much more work than today's human beings. They ate healthy diet just because it was pollution free. There were no industries, mills, or smoke throwing vehicles. There was no fuel burning which is the major cause of pollution. Researches have shown that the ozone layer after the industrial revolution has been damaged far enough and causing diseases like cancer.
