Answer:
The correct answer is:
Getting enough protein is a common concern for vegetarians.
Explanation:
In the text "Protein Power", the author explains how normally vegetarians face some troubles regarding the amount of protein they include in their diet , that has a central rule to avoid the consumption of meat that is the greatest source of protein in a normal diet. Nevertheless, the author explains how, with a little extra effort, it is possible to fulfill all the protein needs, he says, he just pays special attention in order to find the correct balance in his diet.
Wagon trains crossed the Great Plains.
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I would interpret it as how people only look at reality, not what's inside or what they can't see.