For expression in Column I, choose the expression from Column II that completes a fundamental identity. Do not use a calculator.
$\mathbf{I}$
$\frac{\cos x}{\sin x}=$_____
$\mathbf{I}$
A. $\sin ^{2} x+\cos ^{2} x$
B. cot $x$
C. $\sec ^{2} x$
D. $\frac{\sin x}{\cos x}$
E. $\cos x$