It was not until after I had got home that I realized I had not set the burglar alarm in the office.

A On the way home, I suddenly realized that I had forgotten to turn on the burglar alarm in the office.

B Fortunately, I realized that I hadn't set the burglar alarm just before I left for home; otherwise, I would have had to travel all the way back to the office.

C I didn't turn the burglar alarm on before I left the office, but I only became aware of this after I'd arrived home.

D I wish I had realized before I arrived home that I hadn't turned on the burglar alarm in the office; then it would have been easier to go and set it.