Question

Choose the option that correctly combines the sentences.

Someone had knocked at the door. So I opened it. [Advanced]

• A. Someone having knocked at the door, so I opened it.
• B. Someone had knocked at the door and so I opened it.
  
• C. Opening the door, someone has knocked at it.
  
• D. Someone having knocked at the door, I opened it.
  

Answer 1

The correct option is D Someone having knocked at the door, I opened it.

Since one of the actions happened in considerable past, a participle form of that action verb must be used. Therefore, Option D is correct. The rest of the options either use the wrong sequence of events(Option C) or use two conjunctions together(Option A) or do not use a participle( Option A). Therefore, these are incorrect.