Extended Rectangle Type 2
Because the puzzle has exactly one solution, any pattern that would give two can be ruled out.
All candidates have been entered in the empty cells.
Assume 5 is removed from the 2 cells in C. The 7 marked cells in each of row A and row B then use only 1, 3, 4, 6, 7, 8 and 9. Each side must use these 7 digits once. Swap every pair in the same column. Both sides still contain the same digits. Each pair keeps its digits in the same column, and the block contents are unchanged, producing another solution. Because the solution is unique, at least one cell in C contains 5. Remove 5 from each crossed-out cell that sees every cell in C.