Diagram 52 shows two right-angled triangles $ABC$ and $ACD$.
\begin{tikzpicture}[scale=0.8]
\draw (0,0) -- (3,0) -- (0,4) -- cycle;
\draw (0,0) -- (0,-1);
\draw (0,0) -- (-1,0);
\draw (3,0) -- (3.5,0);
\draw (0,4) -- (0,4.5);
\node at (0,0) [below left] {$C$};
\node at (3,0) [below right] {$B$};
\node at (0,4) [above] {$D$};
\node at (3,0) [below] {$B$};
\node at (1.5,0) [below] {$25$};
\node at (0,2) [left] {$45$};
\node at (2.5,0.5) {$x$};
\node at (0.5,0.5) {$y$};
\node at (0,-1) [below] {Diagram 52};
\end{tikzpicture}
Given that $\tan x^\circ = \frac{7}{8}$, find the value of $\cos y^\circ$.
A $\frac{24}{25}$
B $\frac{8}{17}$
C 1
D $1\frac{4}{5}$
