Final answer:
The two RR crossing signs are circular yellow and black, warning of a railroad crossing.
Step-by-step explanation:
The correct answer is c. Circular yellow and black, warning of a railroad crossing.
RR crossing signs are used to warn drivers of an upcoming railroad crossing. They are circular in shape and feature the colors yellow and black. The yellow color stands out and grabs attention, while the black color provides contrast and helps with visibility. This combination of colors is used to quickly convey the message of caution to drivers.
For example, when approaching a railroad crossing, drivers should look out for these signs and be prepared to stop if a train is approaching. They should also check for lights, gates, or other warning signs that may be present. The circular yellow and black RR crossing sign serves as an important visual reminder for drivers to exercise caution and be aware of the potential dangers associated with a railroad crossing.