|  | Years Ended December 31, |
--- | --- | --- | ---
| 2017 | 2018 | 2019
| % | % | %
Operating revenues | 100.0 | 100.0 | 100.0
Operating costs | (81.9) | (84.9) | (85.6)
Gross profit | 18.1 | 15.1 | 14.4
Operating expenses |  |  |
Sales and marketing | (2.8) | (2.6) | (2.6)
General and administrative | (2.8) | (3.2) | (3.6)
Research and development | (9.2) | (8.6) | (8.0)
Expected credit losses | — | (0.3) | (0.4)
Subtotal | (14.8) | (14.7) | (14.6)
Net other operating income and expenses | 1.1 | 3.4 | 3.5
Operating income | 4.4 | 3.8 | 3.3
Non-operating income and expenses | 0.7 | (2.4) | (0.1)
Income from continuing operations before income tax | 5.1 | 1.4 | 3.2
Income tax benefit (expense) | (0.6) | 0.7 | (0.1)
Net income | 4.5 | 2.1 | 3.1
Total other comprehensive income (loss), net of tax | (3.2) | 0.6 | 1.9
Total comprehensive income | 1.3 | 2.7 | 5.0
Net income attributable to: |  |  |
Stockholders of the parent | 6.5 | 5.1 | 5.5
Non-controlling interests | (2.0) | (3.0) | (2.4)
Total comprehensive income attributable to: |  |  |
Stockholders of the parent | 3.3 | 5.7 | 7.4
Non-controlling interests | (2.0) | (3.0) | (2.4)