The faster you travel, the more slowly time passes, which means that time moved quickly when you are walking but more slowly when you are flying in an airplane.

which revision of the sentence uses parallel structure?
a. the faster you travel, the more slowly time passing, which means that time moved quickly when you are walking but more slowly when you are flying in an airplane.
b. the faster you traveled, the more slowly time passes, which means that time moved quickly when you are walking but more slowly when you are flying in an airplane.
c. the faster you travel, the more slowly time passes, which means that time moves quickly when you are walking but more slowly when you are flying in an airplane. d. the faster you travel, the more slowly time passes, which means that time moved quickly when you were walking but more slowly when you are flying in an airplane