If you accept either C or D as the correct revision then you have to accept both of them. Hence the best way to treat the sentence is to ignore both of them.
B is not correct either. You can over punctuate.
That leaves A.
Sometimes the right answer to many things is to do nothing. My dad used to say "Don't fix what isn't broke."
A is the answer <<<<<
Panel discussion because if you take out the moderator out of a panel discussion it becomes a round table.
We can use the next reply: <span>I didn't think I was out of step, but thanks for your opinion. I will try to stay in sync with the band next time!.</span>
Vapor, substance, are all possibilities you can use. Hope this helps :)
