Choose the correct form: Only after the operation ____ the true cause.
- A. they discovered
- B. did they discover
- C. they did discover
- D. discovered they
Explanation
When a restrictive expression such as only after, only then or not until opens the sentence, the subject and auxiliary invert as in a question. Without the inversion the sentence sounds ungrammatical in formal writing. The same applies to little did he know.