Lydia really regrets ________ out of high school．She has really had to struggle to make a living because of that decision．
A.dropB.to dropC.droppingD.dropped在线课程C
解析：
考查非谓语动词。根据下一句可知Lydia“后悔”从高中辍学。regret to do sth表示“遗憾地去做某事”，不符合所给语境。