–Did you ask the guard what happened?
--Yes, he told me all ______ he knew.

–Did you ask the guard what happened?

--Yes, he told me all ______ he knew.

   A. about which      B. which              C. that            D. what

-Did you ask the guard what happened?

-Yes, he told me all ______ he knew.

1. A.
   about which
2. B.
   which
3. C.
   that
4. D.
   what

---Did you ask the guard what happened?
---Yes, he told me all ______ he knew.

1. A.
   about which
2. B.
   which
3. C.
   that
4. D.
   what

 –Did you ask the guard what happened?

--Yes, he told me all ______ he knew.

   A. about which      B. which    C. that            D. what