When is it unsafe to pass on a road?

Answer: B — On a curve or hill where visibility is poor

Passing on a curve or hill where visibility is poor is unsafe primarily because it significantly increases the risk of a collision. When driving in these types of conditions, it's often difficult to see oncoming traffic or potential hazards that may suddenly appear. The inability to accurately judge if the lane is clear ahead due to the limited sight distance creates a dangerous situation. In contrast, passing on a flat stretch of highway generally offers better visibility, allowing drivers to see further ahead and assess whether it is safe to pass. The presence of other cars nearby can be a concern, but it is often a matter of assessing distances and speed rather than an absolute rule against passing. Low-speed zones may also present less risk for passing; however, they still require caution to ensure safety.