The correct answer is D. simple.
An elementary course is a basic course that is designed for beginners. It is typically the first course that students take in a particular subject. Elementary courses are usually very easy and do not require much prior knowledge.
A secondary course is a more advanced course that is designed for students who have already taken an elementary course in the subject. Secondary courses are typically more difficult than elementary
courses and require more prior knowledge.An advanced course is the most difficult type of course. It is designed for students who have already taken a secondary course in the subject. Advanced courses are typically very difficult and require a lot of prior knowledge.
A tough course is a course that is difficult to pass. It is typically a course that requires a lot of work and dedication.
Therefore, the correct answer is D. simple.