According to figure 15-3, which gas makes up 21 percent of earth's atmosphere?

According to the figure, oxygen gas makes up 21 percent of Earth's atmosphere.

Explanation:

This figure is called "pie chart", these type of charts are use to show different categories (types, sorting) of a topic represented as slices whose sum is the 100%.

In this case, the chart topic would be "gases of Earth's atmosphere" and the type of gases represented are: oxygen, nitrogen and water vapor.

Analyzing the chart, oxygen gas makes up 21% of Earth's atmosphere, nitrogen gas makes up 78% and the rest is water vapor. The sum of those gases is the 100%.

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we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).