Which of the following not true?

• A. If u is harmonic, then ∇2u = 0
• B. y = f(x – ct) is the progressive wave-type solution of wave equation.
• C. If ϕ is harmonic in D ⊆ R2, and \(\phi \left( {{x_0}} \right) = \mathop {\max }\limits_{\bar D} \phi ,{x_0} \in D\) , then ϕ is constant in D
• D. If ϕ solves the initial value wave equation in one dimension, then the sum of kinetic energy and potential energy is a function of time.