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:
|  | Number of Firm Commitments (a) | Aircraft Type |
| --- | --- | --- |
|  |  | Number of Firm Commitments (a) |
| Airbus A350 | 45 |  |
| Boeing 737 MAX | 161 |  |
| Boeing 777-300ER | 4 |  |
| Boeing 787 | 18 |  |
| 5 |  | (a) United also has options and purchase rights for additional aircraft. |
Question:
As of 2017, how many Airbus A350 passenger airplanes were ordered by United Airlines?
Answer:
45