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A finite wave train, of an unspecified nature, propagates along the positive X-axis with a constant speed v and without any change of shape. The differential equation among the four listed below, whose solution it must be, is

A. $$\left( {\frac{{{\partial ^2}}}{{\partial {x^2}}} - \frac{1}{{{v^2}}}\frac{{{\partial ^2}}}{{\partial {t^2}}}} \right)\psi \left( {x,\,t} \right) = 0$$

B. $$\left( {{\nabla ^2} - \frac{1}{{{v^2}}}\frac{{{\partial ^2}}}{{\partial {t^2}}}} \right)\psi \left( {\overrightarrow r ,\,t} \right) = 0$$

C. $$\left( { - \frac{h}{{2m}}\frac{{{\partial ^2}}}{{\partial {x^2}}} - ih\frac{\partial }{{\partial t}}} \right)\psi \left( {x,\,t} \right) = 0$$

D. $$\left( {{\nabla ^2} + a\frac{\partial }{{\partial t}}} \right)\psi \left( {\overrightarrow r ,\,t} \right) = 0$$

Answer: Option A


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