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 enroll (stu_num VARCHAR); CREATE TABLE student (stu_fname VARCHAR, stu_num VARCHAR)
### Question:
Find the first names of students who took exactly one class.
### Answer:
SELECT T1.stu_fname FROM student AS T1 JOIN enroll AS T2 ON T1.stu_num = T2.stu_num GROUP BY T2.stu_num HAVING COUNT(*) = 1