由摆线 $\left\{\begin{array}{l}x=a(t-\sin t) \\ y=a(1-\cos t)\end{array},(0 \leq t \leq 2 \pi)\right.$ 和 $x$ 轴所围均匀薄板的重心心坐标是 $($ )
A
$\left(\frac{5}{6} a, a\right)$
B
$\left(a, \frac{5}{6} a\right)$
C
$\left(\frac{5}{6} a, \pi a\right)$
D
$\left(\pi a, \frac{5}{6} a\right)$
E
F