Answer:
The Ethiopian Highlands is a rugged mass of mountains in Ethiopia, situated in northeast part of Africa. It forms the largest continuous area of its elevation in the continent, with little of its surface falling below 1,500 m (4,900 ft), while the summits reach heights of up to 4,550 m (14,930 ft).
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erode means to wear away slowly
I find that the best answer is the following one:
c. determined by factors outside our control
For example, even though life is very hard in the very hot and the very cold regions, we can't control it: so this is an example of a physical environment that is hard to control.
In comparison, it is often easier to describe than our social environment.
Scientists can't be sure about the early atmosphere and can only draw evidence from other sources. For example, volcanoes release high quantities of carbon dioxide. Iron-based compounds are present in very old rocks that could only have formed if there was little or no oxygen at the time.