Which kind of vehicle is gray?
The vehicle is a bus.
    src        edge_attr  dst
0     0             near    2
1     0             near    4
2     0               on    1
3     0   to the left of    2
4     0            below    4
5     2             near    0
6     2               by    0
7     2               on    1
8     2  to the right of    0
9     2             near    3
10    3             near    2
11    4             near    0
12    4            above    0