If $x$ is an angle such that $0< x<90^{\circ}$, which of the following statements is not always true?

Question

$\begin{array}{1 1} (a)\; \cos x > 0\\ (b)\;\cos(-x) >0 \\ (c)\;\cos(x+90^{\circ}) < 0\\(d)\;\cos 2x <0 \end{array} $</span>