An old car with worn-out shock absorbers oscillates with a given frequency when it hits a speed bump. If the driver adds a couple of passengers to the car and hits another speed bump,

(a) Is the car's frequency of oscillation greater than, less than, or equal to what it was before?

(b) Choose the best explanation from among the following:

I. Increasing the mass on a spring increases its period, and hence decreases its frequency.

II. The frequency depends on the force constant of the spring but is independent of the mass.

III. Adding mass makes the spring oscillate more rapidly, which increases the frequency.