Let D = { 16 , 19 , 21 } , E = { 16 , 18 , 19 , 20 } and F = { 15 , 17 , 18 , 19 , 21 } List the elements in the set ( D ? E ) ? F . ( D ? E ) = ? (Use a comma to separate answers as needed List the elements in order with no duplicates.) ( D ? E ) ? F = . (Use a comma to separate answers as needed List the elements in order with no duplicates)