Which of the following is a non-homogeneous equation?

• A. \(\frac{∂^2 u}{∂t^2}-c^2\frac{∂^2 u}{∂x^2}=0\)
• B. \(\frac{∂^2 u}{∂x^2}+\frac{∂^2 u}{∂y^2}=0\)
• C. \(\frac{∂^2 u}{∂x^2}+(\frac{∂^2 u}{∂x∂y})^2+\frac{∂^2 u}{∂y^2}=x^2+y^2\)
• D. \(\frac{∂u}{∂t}-T \frac{∂^2 u}{∂x^2}=0\)