Answer:
The pair of lines in (c) are parallel; so are the pair in (d).
Explanation:
Rule: Two lines are parallel if their slopes are equal. If the slopes of two different lines are equal, then the lines are parallel.
Let's look at these four possible answer choices with that in mind:
a) the slopes are not equal; they are negative reciprocals. These lines are not parallel.
b) The slopes are not equal; one is positive and one is negative. These lines are not parallel.
c) Both of these lines have the slope 4; that is, their slopes are equal. Therefore these two lines are parallel.
d) y = 9 and y = 18 are both horizontal lines, and are therefore parallel. 9 and 18 do not represent slopes; rather, they represent y-intercepts.