Which is the correct way of writing the sentences?<br/><span style="color:#ff0000;"><br/> a. Who did it was a mystery.<br/> b. "Who did it?" was a mystery.<br/><br/> c. What happened that day is not known.<br/> d. "What happened that day" is not known.</span><br/><br/> The sentences are mine.<br/><br/> I had never thought of this before. The problem is that the person who did it is not a mystery. The question is a mystery. I am not sure at all.<br/><br/> Many thanks.