I am very sorry to have kept you waiting for so long.
很抱歉讓您久等了。
HAVE + P.P. (過去式)+Ving 現在完成進行式從過去的狀態一直維持狀態到現在所以有過去式也有進行式
29.I am very sorry to have kept you ____..-阿摩線上測驗