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I think they were talking about science in general.
Single science is complusory for everyone and most people do the exam in year 10, but the people in the lowest set, only do this science for the two years and get tested on it in year 11. Everyone else does double science in year 11 which is slightly harder, then people from the top two sets can choose to do triple science, which gets them a gcse in science, for which you do coursework and learn stuff for the exams in both years, which are tested in year 11 which is slightly harder than the other year 11 stuff. but if you do triple, then it replaces one of your optional subjects such as art of french etc, so I did triple science, art history and geography.