CREATE TABLE table_23157997_13 (mountains_classification VARCHAR(255), youth_classification VARCHAR(255), points_classification VARCHAR(255))
Who was Mountains Classification in the race with Nick Frey as Youth Classification and Tom Zirbel as Points Classification?
    
SQL: SELECT mountains_classification FROM table_23157997_13 WHERE youth_classification = "Nick Frey" AND points_classification = "Tom Zirbel"