Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_20 (service VARCHAR, unit VARCHAR, rank VARCHAR)
### Question:
Name the service for 7th marines rank of sergeant
### Answer:
SELECT service FROM table_name_20 WHERE unit = "7th marines" AND rank = "sergeant"