Table:
|  | Depreciable Lives | As of December 31 |  |
| --- | --- | --- | --- |
|  |  | 2019 | 2018 |
|  |  | (Dollars in millions) |  |
| Land | N/A | $867 | 871 |
| Fiber, conduit and other outside plant(1) | 15-45 years | 24,666 | 23,936 |
| Central office and other network electronics(2) | 3-10 years | 19,608 | 18,736 |
| Support assets(3) | 3-30 years | 7,984 | 8,020 |
| Construction in progress(4) | N/A | 2,300 | 1,704 |
| Gross property, plant and equipment |  | 55,425 | 53,267 |
| Accumulated depreciation |  | (29,346) | (26,859) |
| Net property, plant and equipment |  | $26,079 | 26,408 |
Paragraphs:
(9) Property, Plant and Equipment
Net property, plant and equipment is composed of the following:
(1) Fiber, conduit and other outside plant consists of fiber and metallic cable, conduit, poles and other supporting structures.
2) Central office and other network electronics consists of circuit and packet switches, routers, transmission electronics and electronics providing service to customers.
(3) Support assets consist of buildings, cable landing stations, data centers, computers and other administrative and support equipment.
(4) Construction in progress includes inventory held for construction and property of the aforementioned categories that has not been placed in service as it is still under construction.
We recorded depreciation expense of $3.1 billion, $3.3 billion and $2.7 billion for the years ended December 31, 2019, 2018 and 2017, respectively.
Question:
In which years was the depreciation expense recorded?
Answer: 2019, 2018, 2017