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:
|  | 2017 | 2016 (a) | 2015 | 2014 | 2013 |
| --- | --- | --- | --- | --- | --- |
| Crude oil | 66% | 69% | 62% | 72% | 71% |
| Refining margins | 23% | 20% | 26% | 18% | 19% |
| Other (b) | 11% | 11% | 12% | 10% | 10% |
| Total | 100% | 100% | 100% | 100% | 100% |
| Aircraft fuel expense | 22% | 18% | 22% | 32% | 34% |
Question:
In 2015, what was Alaska airlines largest expense category?
Answer:
Crude oil