What is the differential equation of all parabolas whose directrices are parallel to the x-axis?

(a) d^3x/dy^3 = 0

(b) d^3y/(dx^3 + d^2y/dx^2) = 0

(c) d^3y/dx^3 = 0

(d) d^2y/dx^2 = 0

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Origin of the question is Linear Second Order Differential Equations in portion Differential Equations of Mathematics – Class 12

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