What must be true about the slopes of two perpendicular lines, neither of which is vertical?

The slopes are equal.
The product of the slopes is –1.
One of the slopes must equal 0.
The product of the slopes is 1.

Question

What must be true about the slopes of two perpendicular lines, neither of which is vertical?
 
   The slopes are equal. 
 The product of the slopes is  –1. 
 One of the slopes must equal 0. 
 The product of the slopes is 1.

anonymous · Answer

What must be true about the slopes of two perpendicular lines, neither of which is vertical?
 
   The slopes are equal. 
 The product of the slopes is  –1. 
 One of the slopes must equal 0. 
 The product of the slopes is 1.