If $f ( x ) = \cos x - \sin x$ is decreasing on $[ 0 , a ]$, then the maximum value of $a$ is A. $\frac { \pi } { 4 }$ B. $\frac { \pi } { 2 }$ C. $\frac { 3 \pi } { 4 }$ D. $\pi$
If $f ( x ) = \cos x - \sin x$ is decreasing on $[ 0 , a ]$, then the maximum value of $a$ is\\
A. $\frac { \pi } { 4 }$\\
B. $\frac { \pi } { 2 }$\\
C. $\frac { 3 \pi } { 4 }$\\
D. $\pi$