When Space Probe Voyager 2 Passed by Saturn, Its Speed

Multiple Choice

When space probe Voyager 2 passed by Saturn, its speed increased (but not due to firing its engines) . What must have happened?

A) Saturn must have lost a very tiny bit of its orbital energy.
B) Voyager 2 must have dipped through Saturn's atmosphere.
C) Saturn's rotation must have sped up slightly.
D) Saturn must have captured an asteroid at precisely the moment that Voyager 2 passed by.