Speaker A: Who turned on the lights?

Speaker B:

(A) They are on all the time.

(B) It isn't drak dark outside.

(C) I'll turn them off ooff.

I expected the answer to be "He did.", "She turned on the lights." or something like that. According to the answer key, (A) is the correct answer. However I feel (A) is not a satisfying answer for me. Can anyone explain why (A) is goods?

The situation is that no-one turned on the lights, because they are always on.

This is what answer A means. The speaker could have said

'No-one did, because they are on all the time',

but he thought the underlined part was obvious, so he omitted it. 

Clive