Think of classic Minesweeper game, with the following list of restrictions on the placement of mines:
- C (Connected): All mines are connected via 8-way neighborhood (orthogonal and diagonal).
- Q (Quad): Every 2x2 subgrid contains at least one mine.
- T (Triplet): No three mines are adjacent on a line (row, column, or diagonal). Think of Tic-Tac-Toe.
- O (Outside): All mines are connected to the outside via 4-way neighborhood (orthogonal), and all non-mine cells are connected via 4-way neighborhood.
Here are some examples under each restriction: (X are mines, and . are cells without mines)
| C | Q | T | O |
|---|---|---|---|
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Now, find the unique combination of two restrictions that allow you to open a cell with 100% certainty on a large enough grid. What kind of cell(s) can you open, and what is the minimum size of such a grid?
Note: The answer to this puzzle spoils how to get a specific achievement in 14 Minesweeper Variants (2022) by Artless Games.