AMC 12A 2022 (Problem 12)

Let

M

be the midpoint of

\overline{AB}

in regular tetrahedron

ABCD

. What is

\cos(\angle CMD)

\text{(A)}\;\frac14

\text{(B)}\;\frac13

\text{(C)}\;\frac25

\text{(D)}\;\frac12

\text{(E)}\;\frac{\sqrt3}{2}