CREATE TABLE table_21820 ("SECR No." FLOAT, "SR No." FLOAT, "BR No." FLOAT, "Builder" TEXT, "Date Delivered" TEXT, "Date Withdrawn" TEXT)

Who is the builder for br no. 31779?
SELECT "Builder" FROM table_21820 WHERE "BR No." = '31779'
SELECT "Builder" FROM "table_21820" WHERE "BR No." = '31779'