Select the correct answer. Consider the function

$f(x)=\sqrt[\phantom{2}]{\frac{1-sinx}{1cosx}},$

then the slope of the tangent line to $f$ when $x=0,$ that is

$f^{\text{'}}(0),$

is equal to

$2^{-\frac{1}{2}}$

$-2^{-\frac{3}{2}}$

$-2^{\frac{1}{2}}$

$2^{-\frac{3}{2}}$