Final answer:
Slow driving is dangerous after passing the crest of a hill and rounding a curve due to limited visibility and the potential for accidents due to insufficient reaction time for other drivers.
Step-by-step explanation:
Very slow driving is especially dangerous in both situations mentioned: just after passing the crest of a hill and just after rounding a curve. This is because other drivers may not anticipate a vehicle going much slower than the speed limit, especially in places where visibility is limited such as hills and curves. Aggressive or inattentive drivers may not have sufficient time to react upon discovering a slow vehicle in these situations, leading to an increased risk of accidents.