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. $\left(\pi a, \frac{5}{6} a\right)$