Answer:
carbon dioxide
carbon in the Earth's atmosphere exists in two main forms: carbon dioxide and methane.
Oligarchy: A group of a few people rule a country.
Monarchy: A king or queen rules a country.
Constitutional Monarchy: A king or queen rules the country but has limited powers from a constitution.
Dictatorship: One person is in absolute complete power.
These forms of government are all similar in one way, there is one or a group of people with most of the power. This is unlike other governments which have different bodies of powers that keep each other in checks and balances.
If you are to choose Dictatorship and Oligarchy. Above are two differences.
Similarities are complete control.
Answer: El Salvador
Explanation: El Salvador has a population density of 303 people per square kilometer (785 per square mile).