—You should have thanked her before she left.—I meant but when I was leaving I couldn’t

问题补充：

—You should have thanked her before she left.

—I meant , but when I was leaving I couldn’t find her anywhere.

A. to do

B. to

C. doing

D. to have done

答案：

A. to do

(答案→)B. to

C. doing

D. to have done

解析：这是一组对话。上文，你应在走之前感谢她。下文，我打算这样做，但是当我要离开时去找不到她。这是动词不定式的省略形式，后面的do sth一起省略，这里必须要用mean to do而不是doing（前者是“打算做某事”，后者“意思是，意味着”，意思不对）。A的do是一个实义动词，需要加宾语sth，动词不定式的省略是到to为止的（有时表示过去要加上have），后面的do sth要一起省。