Billboard R&B Number-One Songs, 1970 to 1989 Quiz

Every song to finish a year at number one on Billboard's R&B/hip-hop year-end chart between 1970 to 1989: 20 published chart rows, one per available year, taken from the same published table as the complete run. This is the era cut, for when the full set from 1946 is more than you want in one sitting. Name each from its year, with the artist and the title's first letter available on the easier settings, or sort a selection into the order they led the chart. Parsed verbatim from the published year-end table; no answer is named here.