Can one say<span style="color:#ff0000;"><br/> a. He was singing, which was what he had to do.<br/> b. He was singing, which was what he had to be doing.<br/><br/> c. He was singing, which was what he was supposed to do.<br/> d. He was singing, which was what he was supposed to be doing.</span><br/> ?<br/><br/><br/> The tenses match in (b) and (d), but not in (a) and (c). I suppose it would be safer to use (b) and (d), but are (a) and (c) incorrect?<br/><br/> Many thanks.