The Well-Tempered Clavier, Book II, BWV 870-893 - J.S. Bach: The Well-Tempered Clavier, Book II, BWV 870-893 - Prelude No. 21 in B-Flat Major, BWV 890 (该井平均律，第二册，作品870 - 893 - 第二十一降B调前奏曲)