Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_21820 ( "SECR No." real, "SR No." real, "BR No." real, "Builder" text, "Date Delivered" text, "Date Withdrawn" text )

### Question ###
Who is the builder for br no. 31779?

### Accurate SQL ###

SELECT "Builder" FROM table_21820 WHERE "BR No." = '31779'