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 |
| --- | --- | --- | --- | --- |
| 2019 | 4,292 | $8,953 | $2.09 | 23% |
| 2018 | 4,137 | $9,307 | $2.25 | 24% |
| 2017 | 3,978 | $6,913 | $1.74 | 20% |
Question:
How many gallons of fuel were consumed by United airlines in the year 2018 ?
Answer:
4,137