Answer: Because Point A is negative and the same distance below 0 on the number line as Point B is above it, Point B is both positive and the opposite of Point A.
According to Barbara's plot, the number line A, is above sea level, therefore, it's a positive line, in positive range.
On the other hand, the number line B has the same distance but in the opposite direction, is downwards the zero point. So, this means the B line is below sea level, because is in positive range. That's what we can deduct from this, because the characterization Barbara is doing is related to sea level.
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