A scientist crossed two tall pea plants which had round seeds. Out of the offspring, 600 plants had round seeds and 200 had wrinkled seeds. What percentage of the pea plants had round seeds?

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled