Who was Mountains Classification in the race with Nick Frey as Youth Classification and Tom Zirbel as Points Classification?
CREATE TABLE table_23157997_13 (
    mountains_classification VARCHAR,
    youth_classification VARCHAR,
    points_classification VARCHAR
)
SELECT mountains_classification FROM table_23157997_13 WHERE youth_classification = "Nick Frey" AND points_classification = "Tom Zirbel"
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: Who was Mountains Classification in the race with Nick Frey as Youth Classification and Tom Zirbel as Points Classification? ### Input: CREATE TABLE table_23157997_13 (
    mountains_classification VARCHAR,
    youth_classification VARCHAR,
    points_classification VARCHAR
) ### Response: SELECT mountains_classification FROM table_23157997_13 WHERE youth_classification = "Nick Frey" AND points_classification = "Tom Zirbel"