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 <<<<<
The answer is <span>A. Several of the speakers suggested that much remained to be done.
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I'd say it is D, but I'm not 100% sure. I'd wait to see if anyone else thinks so too.
