Originally, I designed it by gating the clock input in a special way. The question was then reworded so that clocks could not be gated. To get clarification for the question, I asked if I could only use D-Flip flops and nothing else. Was told yes, but realized after the interview that the interviewer probably meant to say that using basic logic functions(AND/OR) was allowed, thus making it impossible to answer the question if I was stuck with the idea that only D-Flip Flops could be used.