Step 1: Observe the ratio:
$x_2 = \sqrt2\,x_1$ for every t, because $x_2$ is just $\sqrt2$ times the expression of $x_1$.
Step 2: Meaning:
One function is a constant multiple of the other, so they reach maximum and zero together. The phase difference is zero (A).
Final Answer:
Zero.
\[ \boxed{0} \]