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1. Question
The Terrestrial Planets
A The terrestrial planets are quite different from the giants. In addition to being much smaller, they are composed primarily of rocks and metals. B These, in turn, are made of elements that are less common in the universe as a whole. C The most abundant rocks, called silicates, are made of silicon and oxygen, and the most common metal is iron. D We can tell from their densities that Mercury has the greatest proportion of metals (which are denser) and the Moon has the lowest. Earth, Venus, and Mars all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be oxidized.
When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.
Differentiation is the process by which gravity helps separate a planet’s interior into layers of different compositions and densities. The heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust. Later, when the planet cools, this layered structure is preserved. In order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than 1,800 F.
Q. In paragraph 1 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?
The terrestrial planets, as they are known, consist of the four innermost planets of our solar system – namely, Mercury, Venus, Earth, and Mars.
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2. Question
Metabolism and Body Weight
Our body weight is affected by a number of factors, including gene-environment interactions, and the number of calories we consume versus the number of calories we burn in daily activity. If our caloric intake exceeds our caloric use, our bodies store excess energy in the form of fat. If we consume fewer calories than we burn off, then stored fat will be converted to energy. A Our energy expenditure is obviously affected by our levels of activity, but our body’s metabolic rate also comes into play. B A person’s metabolic rate is the amount of energy that is expended in a given period of time, and there is tremendous individual variability in our metabolic rates. C People with high rates of metabolism are able to burn off calories more easily than those with lower rates of metabolism. D
We all experience fluctuations in our weight from time to time, but generally, most people’s weights fluctuate within a narrow margin, in the absence of extreme changes in diet and/or physical activity. This observation led some to propose a set-point theory of body weight regulation. The set-point theory asserts that each individual has an ideal body weight, or set point, which is resistant to change. This set-point is genetically predetermined and efforts to move our weight significantly from the set-point are resisted by compensatory changes in energy intake and/or expenditure.
Some of the predictions generated from this particular theory have not received empirical support. For example, there are no changes in metabolic rate between individuals who had recently lost significant amounts of weight and a control group. In addition, the set-point theory fails to account for the influence of social and environmental factors in the regulation of body weight. Despite these limitations, set-point theory is still often used as a simple, intuitive explanation of how body weight is regulated.
Q. In paragraph 3 there is a missing sentence. Look at the four squares [A, B, C, D] that indicate where the sentence could be added. Where would the sentence best fit?
This variability in metabolic rate is due to a variety of factors including age, gender, physical activity, hormone function, and muscle-to-fat ratio.
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