What Allowed the Voyager 2 Spacecraft to Make a "Tour

What allowed the Voyager 2 spacecraft to make a "tour" of all four of the jovian planets in the late 1970's and the 1980's?

A) NASA had developed a completely new kind of rocket that could send the craft from planet to planet
B) the four planets were approximately aligned on one side of the Sun and we used the gravity of each planet to speed up the spacecraft to get to the next one in its path
C) the spacecraft stopped off to collect fuel on the satellites of each planet before proceeding to the next one
D) we used laser beams to propel the spacecraft into the outer solar system, where sunlight is dim
E) you can't fool me, no spacecraft has ever explored four different planets