如图所示，边长a=20cm的正方形挖去边长b=10cm的正方形，y轴是其铅垂对称轴，则其形心的y坐标等于(  )

A、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/2fc22e7c-81cf-4e1c-b11e-22c7c2b4fdf5.png"title="ex_1397115926652_0.png" alt="ex_1397115926652_0.png"/>

B、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/32d7096d-fd88-4792-b6a1-a2167170c883.png"title="ex_1397115949218_0.png" alt="ex_1397115949218_0.png"/>

C、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/ec3104a2-9ad7-4826-8058-f6c24351b161.png"title="ex_1397115970923_0.png" alt="ex_1397115970923_0.png"/>

D、<IMG src="https://s3.cn-north-1.amazonaws.com.cn/qingshuxuetang/staticQuestionImg/e8981839-1ab9-4305-850f-fa969c96ff30.png"title="ex_1397115992985_0.png" alt="ex_1397115992985_0.png"/>