The most abundant greenhouse gas in Earth's atmosphere is ________.
A. methane
B. oxygen
C. carbon dioxide
D. tropospheric ozone
Answer: C
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The economic activities not a part of the official record and that take place outside official systems of regulation or remuneration make up the ________ sector of the economy and tend to be an important part of urban economic life around the world
A) informal B) underground C) black market D) market
The surface between the saturated zone (below) and the unsaturated zone (above) is called the:
a) water table b) hard pan c) saturated lid d) recharge zone e) head level
This figure shows three blocks in water. Which of these materials is the densest? 
A. block 1 B. block 2 C. block 3 D. the water 4
Which statement accurately describes hydrogen?
A. Hydrogen is fairly rare in the universe. B. Hydrogen is abundant in the atmosphere as a free gas. C. Hydrogen has a high net energy yield. D. Hydrogen gas is a fuel produced by using other forms of energy. E. Hydrogen use in fuel cells emits mercury pollution.