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_20999 ( "Year" real, "Mens Open" text, "Womens Open" text, "Mixed Open" text, "Mens u20" text, "Womens u20" text, "Senior Mixed" text, "Mens 30" text, "Womens 30" text, "Mens 35" text, "Womens 35" text, "Mens 40" text, "Womens 40" text, "Mens 45" text, "Mens 50" text )

### Question ###
What was the mens open when the mens u20 was Qld Country Rustlers def Southern Suns?

### Accurate SQL ###

SELECT "Mens Open" FROM table_20999 WHERE "Mens u20" = 'Qld Country Rustlers def Southern Suns'