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 Consumed (in millions) | Fuel Expense (in millions) | Average Price Per Gallon | Percentage of Total Operating Expense |
| --- | --- | --- | --- | --- |
| 2017 | 3,978 | $6,913 | $1.74 | 20% |
| 2016 | 3,904 | $5,813 | $1.49 | 18% |
| 2015 | 3,886 | $7,522 | $1.94 | 23% |
Question:
How much gallon of fuel has been consumed by United Airlines in 2016?
Answer:
3,904