user: Question: What is the wheel arrangement with 1 quantity preserved?
Columns: ['Class', 'Wheel arrangement', 'Fleet number(s)', 'Year made', 'Quantity made', 'Quantity preserved']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['4-6-0 — ooOOO', '4-6-0 — ooOOO', '4-6-0 — ooOOO', '4-6-0 — ooOOO', '4-6-0 — ooOOO', '4-6-0 — ooOOO']
SQL:

assistant: SELECT "Wheel arrangement" FROM "Table" WHERE "Quantity preserved" = '1';