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:
| Route Name | Direction | Termini | Junctions | Length | Population Area | Remarks |
| --- | --- | --- | --- | --- | --- | --- |
| SH 2 | South North | Laredo, Texas La Salle County | none | miles (km) | Laredo Webb | Replaced by US 81 |
| SH 4 | South North | Zapata County Dimmit County | none | miles (km) | Laredo Webb | Replaced by US 83 |
| US 81 | South North | Laredo, Texas La Salle County | none | miles (km) | Laredo Webb | Replaced by I-35 |
| US 83 Bus. | South North | Gateway to the Americas Int'l Bridge I-35 | I-35 US 83 | miles (km) | Laredo | Replaced by BSI-35 |
| SH 96 | East West | Duval County Laredo | none | miles (km) | Las Lomas Laredo | Replaced by US 59 |
| SH 202 | East West | Duval County Laredo | none | miles (km) | Las Lomas Laredo | Replaced by US 96 |
| LP 420 | South North | Gateway to the Americas Int'l Bridge I-35 | I-35 US 83 | miles (km) | Laredo | Replaced by US 83 Bus. |
| FM 863 | East West | Duval County US 83 | US 83 | miles (km) | None | Replaced by SH 44 |
Question:
What unit of length is being used for the route with "replaced by us 81" in their remarks section?
Answer:
miles (km)