题目

— Did you happen to see _______ black and _______ white cat? — Are they missing? I told you to take care of them. A. a; 不填     B. the; 不填 C. the; the      D. a; the 答案：C18．I ___________ frustrated when I wasn't sure of the correct answer．（　　）A．may beB．used toC．used to beD．use to be