the end they arrived in the city the morning of June 20 .A. in; at B. By; on C. At;

问题补充：

the end, they arrived in the city the morning of June 20, .

A. in; at

B. By; on

C. At; in

D. In; on

答案：

A. in; at

B. By; on

C. At; in

(答案→)D. In; on

解析：句意：最终我们于6月20日的早上到达了那座城市。In the end 最后，最终；表示具体某一天的早上或晚上都用介词on，故选D。