  Recently, an indicative evidence of a stochastic process, reported by the
NANOGrav Collaboration based on the analysis of 12.5-year pulsar timing array
data which might be interpreted as a potential stochastic gravitational wave
signal, has aroused keen interest of theorists. The first-order cosmological
confinement/deconfinement phase transition at the QCD scale could be one of the
cosmological sources for the NANOGrav signal. If the phase transition is flavor
dependent and happens sequentially, it is important to find the dominant phase
transition producing the gravitational waves that match the NANOGrav signal
during the evolution of the universe. In this paper, we would like to
illustrate that the NANOGrav signal could be generated from
confinement/deconfinement transition in either heavy static quarks with a zero
baryon chemical potential, or quarks with a finite baryon chemical potential,
or a pure gluon system. Moreover, the quark confinement as compared with the
gluon confinement is more likely to be the source for the NANOGrav signal
regardless of whether the chemical potential is finite or not. Future
observation will help to distinguish between different scenarios.
