Based on the clue 'a downtown area of this major city is named for th...', what is the answer?

Based on the clue 'a doughnut swiss cheese the ozone layer', what is the answer? Based on the clue 'a dover delaware museum depicts the history of thi...', what is the answer? Based on the clue 'a dowel pin is used by these health professionals ...', what is the answer? Based on the clue 'a down filled quilt used as a comforter', what is the answer? Based on the clue 'a dozen us counties are named for this 19th centur...', what is the answer? Based on the clue 'a dpt shot protects against diptheria tetanus per...', what is the answer? Based on the clue 'a drain hose on this such as a kenmore may connect...', what is the answer? What is referred to by 'a drain'?