Give the student's ID of students with 2.5 GPA and enrolled in C for Programmers.
SELECT T2.student_id FROM student AS T1 INNER JOIN registration AS T2 ON T1.student_id = T2.student_id INNER JOIN course AS T3 ON T2.course_id = T3.course_id WHERE T3.name = 'C for Programmers' AND T1.gpa = 2.5