We rushed to the football court, only to see a sign _____ into the ground reading PERIODIC MAINTENANCE (定期维修)
A.knockedB.knockingC.to be knockedD.being knocked在线课程A
解析：
考查非谓语动词。句意“我们冲到足球场，结果只看到一个扎进地面的标志牌，写着‘PERIODIC MAINTENANCE’。”sign和knock是动宾关系，一个用过去分词，表示被动和完成。相当于which was knocked into...。故选A。