Let \( u = u(x, t) \) be the solution of
\[
\frac{\partial u}{\partial t} = \frac{\partial^2 u}{\partial x^2}, \quad 0<x<1, \, t>0,
\]
with boundary conditions \( u(0, t) = u(1, t) = 0 \) and initial condition \( u(x, 0) = \sin(\pi x) \). Define
\[
g(t) = \int_0^1 u^2(x, t) \, dx.
\]
Which one of the following is correct?