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:
| Bridge name | Location | Year completed | Total length | Carries |
| --- | --- | --- | --- | --- |
| Old Youngs Bay Bridge | Astoria, Oregon | 1921 | 1,766.2 feet (538.3 m) | U.S. Route 101 |
| Oregon City Bridge | Oregon City, Oregon | 1922 | 745 feet (227 m) | Oregon Route 43 |
| Dry Canyon Creek Bridge | near Rowena, Oregon | 1922 | 101.1 ft | U.S. Route 30 |
| Winchester Bridge | Winchester, Oregon | 1923 | 884 feet | Oregon Route 99 |
| Lewis and Clark River Bridge | Astoria, Oregon | 1924 | 828 feet | U.S. Route 101 |
| Ellsworth Street Bridge | Albany, Oregon | 1925 | 1,090 feet | U.S. Route 20 |
| Rocky Creek Bridge | Lincoln County, Oregon | 1927 | 360 feet | U.S. Route 101 |
| Depoe Bay Bridge | Depoe Bay, Oregon | 1927 | 312 feet | U.S. Route 101 |
| Crooked River High Bridge | Jefferson County, Oregon | 1926 | 464 feet | U.S. Route 97 |
| Big Creek Bridge | Lane County, Oregon | 1931 | 180 feet | U.S. Route 101 |
| Ten Mile Creek Bridge | near Yachats, Oregon | 1931 | 180 feet | U.S. Route 101 |
| Wilson River Bridge | Tillamook County, Oregon | 1931 | 180 feet | U.S. Route 101 |
| Rogue River Bridge | Grants Pass, Oregon | 1931 | 550 feet | Redwood Highway |
| Cape Creek Bridge | near Heceta Head | 1932 | 619 feet (188.6 m) | U.S. Route 101 |
| Isaac Lee Patterson Bridge | Gold Beach, Oregon | 1932 | 1,898 feet (578.5 m) | U.S. Route 101 |
| John McLoughlin Bridge | Oregon City, Oregon | 1933 | 720 feet | Oregon Route 99E |
| Umpqua River Bridge | Reedsport, Oregon | 1936 | 2,206 feet | U.S. Route 101 |
| Siuslaw River Bridge | Florence, Oregon | 1936 | 1,568 feet | U.S. Route 101 |
| Alsea Bay Bridge | Waldport, Oregon | 1936 | 3,028 feet | U.S. Route 101 |
| Yaquina Bay Bridge | Newport, Oregon | 1936 | 3,223 feet (982 m) | U.S. Route 101 |
| Coos Bay Bridge | North Bend, Oregon | 1936 | 5,305 feet (1.6 km) | U.S. Route 101 |
Question:
how many bridges were built in 1931?
Answer:
4