Under what conditions are roadways most slippery during rainfall?

Answer: B — When it first begins to rain

The roadway is most slippery when it first begins to rain due to the combination of oil and water on the surface. During dry conditions, oils from vehicles accumulate on the road. When it starts to rain, the initial drops mix with these oils, creating a slick surface that can significantly reduce traction. This creates a hazardous driving environment, particularly in the first few minutes of a rainstorm. As rainfall continues, the oils are eventually washed away, leading to improved traction. Therefore, after some time, the slipperiness decreases as more rain falls and the road is cleared of the accumulated oils and debris. Additionally, while freezing temperatures can create icy conditions and heavy downpours might cause flooding, they do not have the same immediate slippery effect as the onset of rain.