Twin studies document similarities and differences between twins. when two twins raised under very circumstances display similar personality traits, that suggests a strong influence on personality. critics of twin studies suggest that some documented similarities between twins are based on the fact that the twins are still the same age and may have some because of it.

The correct answers are: "identical", "different", "genetic", and "coincidental"

Explanation:

Twin studies are helpful to investigate the degree of influence that inheritance and environmental factors have in traits such as personality. Two identical twins raised under very different circumstances are very interesting cases of study, because if they show similar personality traits it would suggest that genetic plays an important role in the development of personality. However, critics of twin studies argue that identical twins obviously are the same age, therefore it could be coincidental that they could share some personality traits.

the answer is:

carbon dioxide