i must work hard on english.短文改错,为什么要把ON改成AT?

i must work hard on english.短文改错,为什么要把ON改成AT?
i must work hard on english.
短文改错,为什么要把ON改成AT?
The young man came riding full speed down the road on his bicycle._______it was!
A how dangerous the scene
B what dangerous a scene
C how dangerous scene
D what a dangerous scene