The front and rear suspension systems on a car can move independently, but are coupled because they are both attached to the car body. The car body has two independent degrees of freedom (DOF). The system is modeled as a two-DOF system where the front and rear suspensions are represented by two independent springs.

What type of system is the car suspension modeled as?
a. Two-DOF system
b. Four-DOF system
c. Coupled system
d. Independent system