Ostriches are the fastest running birds and can reach speeds of approximately 70 km/h (43 mph) in short bursts, making them the fastest two-legged animals on Earth. They can maintain a steady speed of about 50 km/h (31 mph) for longer distances, which is still faster than most land animals. Their powerful legs and unique foot structure with two toes contribute to their remarkable speed. Option C (80 km/h) is close to the cheetah's speed (the fastest land animal), while options A and D are either exaggerated or too conservative.