The product of the slopes is negative, if one line is slanted “downwards” and the other one “upwards”. Insufficient, try 1 and 6 (negative and positive slopes) and then try 6 and 6 (negative slopes) , 2 and 2 (positive slopes) , -5 and -2 (positive slopes).
Insufficient, try 1 and -1 (positive slopes), 4 and -6 (positive and negative slopes).
Both: The line which has negative y intercept, and one of its points lie in the first quadrant, must have positive x intercept and a positive slope .
The line which has positive y intercept and positive x intercept must have negative slope.
Thus the product of the slopes is negative. Suff. C