---You may find the key to the maths problem on page 107 .

   ---Ah, it’s so simple . I wonder why I         of that .

         A. hadn’t thought            B. haven’t thought

         C. didn’t think                     D. wasn’t thinking

---You may find the key to the maths problem on page 107.

   ---Ah, it’s so simple. I wonder why I ________ of that.

A. didn’t think     B. haven’t thought  C. wasn’t thinkingD. hadn’t thought

---You may find the key to the maths problem on page 107.

---Ah, it’s so simple. I wonder why I ______ of that.

1. A.
   didn’t think
2. B.
   haven’t thought
3. C.
   wasn’t thinking
4. D.
   hadn’t thought