What usually causes locked wheel skids?

Answer: C — Braking too hard at a high speed

Locked wheel skids typically occur when a driver applies the brakes too forcefully while traveling at a high speed, leading to a loss of traction. When the wheels lock up, they stop rotating, which can cause the vehicle to skid uncontrollably in the direction it's moving rather than allowing the driver to steer it effectively. This situation is exacerbated at higher speeds because the forces at play are much greater, making it harder to regain control once the wheels have locked. In contrast, sudden acceleration could lead to loss of control but does not inherently cause wheels to lock. Similarly, turning too sharply can lead to loss of traction and skidding, but it’s not directly the same as locking the wheels. Driving on wet roads may reduce traction, but it wouldn't directly cause a locked wheel skid unless combined with hard braking. Therefore, the primary cause of locked wheel skids is indeed braking too hard at high speed, which is why this choice is correct.