How Does the Maser Technique Work in Providing a Direct

How does the maser technique work in providing a direct distance measurement to galaxies at intermediate distances (5-10 Mpc range) ?

A) Velocity measurements of maser sources on either edge of the galaxy from Doppler shift of the very narrow maser spectral lines are combined with very precise angular motions of equivalent sources across the front of the galaxy, measured with radio interferometry, to calculate distances.
B) Velocity measurements of maser sources on either side of the galaxy from Doppler shift of the very narrow maser spectral lines are combined with observations of rates of intensity flickering of these small sources to calculate distances.
C) Velocity measurements of maser sources on either side of the galaxy from Doppler shift of the very narrow maser spectral lines are combined with very precise angular separation between the sources and the centers of the galaxies by radio interferometry to calculate distances.
D) Very precise angular motions of sources in front of the galaxy, measured with radio interferometry, are combined with observations of the rates of flickering of intensity of these small sources to calculate distances.