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_2911781_3 ( constructor VARCHAR, grand_prix VARCHAR )

### Question ###
What was the constructor in the Belgian Grand Prix round?

### Accurate SQL ###

SELECT constructor FROM table_2911781_3 WHERE grand_prix = "Belgian grand_prix"