The national exams (GCE 'O' and 'A' Levels) are standardized. School prelim papers in 2021 were intentionally harder than the actual GCE to toughen students up. If you only train on the brutal difficulty of the 2021 cohort, you risk over-analyzing simple questions on the actual exam day.
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The power of is that you have multiple answer keys. Compare your working to 2-3 different school answer keys to understand alternative solving methods. The national exams (GCE 'O' and 'A' Levels) are standardized