Question: Who was Mountains Classification in the race with Nick Frey as Youth Classification and Tom Zirbel as Points Classification?
Schema: create table table_23157997_13 (mountains_classification varchar, youth_classification varchar, points_classification varchar, PRIMARY KEY (mountains_classification))
SQL: select mountains_classification from table_23157997_13 where youth_classification = "nick frey" and points_classification = "tom zirbel"