There were 50 students in a school. The school has three courses, A, B, and C. Students can take 0, 1, 2 and 3 courses. The numbers of students that took A, B, and C were 22, 25, and 33, respectively. 11 students took both A and B. 20 students took both B and C, and 19 students took both A and C. 9 students took all three courses. How many students didn't take any course?
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正确答案: E