Help solve the unsolved questions

Some past-paper questions here have no worked solution yet. Answer them, show the step that decides it, and help get them checked and published for everyone.

• Questions without a solution yet. They come from past papers, most of which have no official answer key.
• Answer and show the deciding step. A letter alone cannot be checked; a few lines of working can.
• Answers stay independent. You never see what others chose, so nobody copies the crowd — and the most popular option is often the trap.
• Checked before anything is published. An answer goes on the site only once it is confirmed by an independent source and a worked solution is written.