In the ancient world, why did civilizations flourish around great river valleys?
(a) river valleys protected citizens from natural disasters.
(b) river valleys provided an ideal environment for agriculture.
(c) river valleys sheltered citizens from harsh weather.
(d) river valleys contained vast mineral resources.

Rewrite the passage to improve expression.

Rick read the following argument: although the gas helium is the second most abundant element in the known universe, its supplies on earth are limited. most helium comes either from mining minerals or as a byproduct of natural gas. although helium is in low supply, the demand for it is quite high. it can be used for a variety of medical purposes, including the treatment of breathing problems such as asthma. helium also has heart and nerve protective properties, and doctors are beginning to use it in surgery. yet current laws make recycling helium unprofitable. for the sake of our health, those laws must change. which statement would be the most appropriate in a conclusion to this argument? why? a. “these laws are unlikely to change, so we must take steps to manufacture helium on our own.” this statement makes the logical leap that if we can't recycle helium, we should create it. b. “until human disease is a thing of the past, we cannot support laws that put helium supplies at risk.” this restates the claim of this argument, which is that we should do everything in our power to treat human diseases. c. “if there is any practical use for helium other than medicine, laws should change to protect current supplies of the gas.” this sentence summarizes one of the major points of the argument, which is that helium reserves are in low supply. d. “human health depends on helium, and therefore it is necessary to change the laws that make it hard to acquire.” this restates the claim of the argument and also summarizes the supporting evidence that illnesses can be treated by helium.

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