What is the smallest quantity having a Class to 1928 of BDi-21?
CREATE TABLE table_64028 (
    "Class to 1928" text,
    "Class from 1928" text,
    "Seats" text,
    "Quantity" real,
    "Year(s) of Manufacture" text,
    "Remarks" text
)
SELECT MIN("Quantity") FROM table_64028 WHERE "Class to 1928" = 'bdi-21'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: What is the smallest quantity having a Class to 1928 of BDi-21? ### Input: CREATE TABLE table_64028 (
    "Class to 1928" text,
    "Class from 1928" text,
    "Seats" text,
    "Quantity" real,
    "Year(s) of Manufacture" text,
    "Remarks" text
) ### Response: SELECT MIN("Quantity") FROM table_64028 WHERE "Class to 1928" = 'bdi-21'