Let the inclination angle of the approach bridge be α and the gravity of the vehicle be g, and decompose the gravity of the vehicle into two components, which are downward along the bridge deck and perpendicular to the bridge deck, then

According to mathematical knowledge, the downward component along the bridge deck is F 1=Gsinα, and the vertical component is F2 = GCOS α.

When the approach bridge is long, if α is small, F 1 is small, that is, the component of gravity parallel to the approach bridge surface decreases.

However, the pressure of the car on the bridge deck is equal to F2=Gcosα. α hours, F2 is large, and the positive pressure of the car on the bridge deck is large.

Therefore, the main purpose of making the approach bridge so long is to reduce the component of vehicle gravity parallel to the approach bridge surface. So D is right and ABC is wrong.

Therefore, choose: d.