The differential equation of all non-horizontal lines in a plane is
(A) d2y/dx2 = 0
(B) d2x/y2 = 0
(C) dy/dx = 0
(D) dx/dy = 0
Answer is (B) d2x/dy2 = 0
The general equation of all non-horizontal lines in xy-plane is
x = my + c
⇒ dx/dy = m [differentiating w.r.t. y]
⇒ d2x/dy2 = 0 [differentiating w.r.t. y]
