  We derive a first-order, stable and causal, relativistic hydrodynamic theory
from the microscopic kinetic equation using the gradient expansion technique in
a general frame. The general frame is introduced from the arbitrary matching
conditions for hydrodynamic fields. The interaction is introduced in the
relativistic Boltzmann equation through the momentum-dependent relaxation time
approximation (MDRTA) with the proposed collision operator that preserves the
conservation laws. We demonstrate here for the first time that not only the
general frame choice, but also the momentum dependence of microscopic
interaction rate, captured through MDRTA, is imperative for producing the
essential field corrections that give rise to a causal and stable first-order
relativistic theory.
