CREATE TABLE table_name_88 (class_from_1928 VARCHAR, class_to_1928 VARCHAR)

Which class from 1928 had a class to 1928 of BDi-21?
SELECT class_from_1928 FROM table_name_88 WHERE class_to_1928 = "bdi-21"
SELECT "class_from_1928" FROM "table_name_88" WHERE "bdi-21" = "class_to_1928"