Solve the system of equations using Cramer's Rule if it is applicable. If Cramer's Rule is not applicable, say so.\\
$\begin{cases} x + y - z = 10\\ 2x - y + z = 5\\ x + 2y - 5z = 21 \end{cases}$\\Determine the fractions obtained while using Cramer's rule. Select the correct choice below and, if necessary, fill in the answer boxes to complete your choice.\
A. Cramer's Rule is applicable. $D = \Box$, $D_x = \Box$, $D_y = \Box$, $D_z = \Box$\
B. Cramer's Rule is not applicable.