On a two-dimensional graph representing two variables: O a. a negative slope of a curve means the two variables are positively related.
O b. a pojitive slope of a curve means the variables are negatively related.
O c. à line that is vertical has a zero slope.
O d. a line that is horizontal has a zero slope.

O d. a line that is horizontal has a zero slope.

Explanation:

When the line moves horizontally, then it makes no or zero slope because it does not move upward or downward direction, it only moves from left to right direction while on the other hand, when the line moves vertically, it makes slope.  A negative slope means that two variables are negatively related whereas, a positive slope means that two variables are positively related.

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