Pioneer 10 and 11 Carried Plaques Intended to Show Any

Pioneer 10 and 11 carried plaques intended to show any alien civilization that found them the location of the solar system in the Galaxy. The location was marked relative to various pulsars (as seen in Figure 20-10b in the text) . Which one of these is the BEST reason to choose pulsars for this purpose?

A) Pulsars can be detected a long way across the Galaxy and thus presumably would be known to the aliens.
B) Pulsars can be identified uniquely by the frequencies of the radiation they emit.
C) Pulsar pulse rates are constant, never changing.
D) Pulsars form a more-or-less uniform ring around the galactic center (like the globular cluster halo) and thus provide a good coordinate system.