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_23157997_13 (mountains_classification VARCHAR, youth_classification VARCHAR, points_classification VARCHAR)

### Question ###
Who was Mountains Classification in the race with Nick Frey as Youth Classification and Tom Zirbel as Points Classification?

### Accurate SQL ###

SELECT mountains_classification FROM table_23157997_13 WHERE youth_classification = "Nick Frey" AND points_classification = "Tom Zirbel"