Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Year | Gallons | Average  Price per Gallon | Aircraft  Fuel Expense | Percent of Total Operating Expenses |
| --- | --- | --- | --- | --- |
| 2017 | 4,352 | $1.73 | $7,510 | 19.7% |
| 2016 | 4,347 | 1.42 | 6,180 | 17.7% |
| 2015 | 4,323 | 1.72 | 7,456 | 21.4% |
Question:
which year had the lowest aircraft fuel expense for American Airlines between 2015 and 2017
Answer:
2016