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Answer:
a) a, b
b) d
Explanation:
A line with positive slope goes up to the right. The right end is higher than the left end. Lines a and b have positive slope.
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A line with negative slope goes down to the right. The right end is lower than the left end. Line d has negative slope.
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Vertical line y is said to have "undefined" slope. Horizontal lines c and x have zero slope (neither positive nor negative).