As Time Progresses,the Pulse Rate for Most Solitary Pulsars

Multiple Choice

As time progresses,the pulse rate for most solitary pulsars:

A) varies periodically as the neutron star undergoes periodic expansions and contractions.
B) slows down since rotational energy is being used to generate the pulses.
C) speeds up as the neutron star slowly contracts under gravity.
D) remains absolutely constant; pulsars provide ideal frequency standards, or clocks.