It seems like you're looking for specific words to fill in the blanks in your sentences. Here are the most appropriate options:
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The B European part of Russia is most heavily populated. The areas that have the highest population rates have the best climates for D agriculture.
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C Siberia has a low population density; one of the reasons is because of the A permafrost. The area has limited vegetation, and the permafrost makes construction of roads and buildings difficult.
So, filling in the blanks with these options, the sentences would read:
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The European part of Russia is most heavily populated. The areas that have the highest population rates have the best climates for agriculture.
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Siberia has a low population density; one of the reasons is because of the permafrost. The area has limited vegetation, and the permafrost makes construction of roads and buildings difficult.