In the Verification of the Hubble Law for the Expansion

Multiple Choice

In the verification of the Hubble law for the expansion of the universe and the determination of the constant H₀, the greatest difficulty has been

A) allowance for gravitational redshifting of light by high gravitational fields at distant galaxies.
B) accurate determination of distances to very distant galaxies.
C) measurement of recession velocities of distant galaxies by Doppler shift.
D) the identification of distant objects as galaxies, rather than very bright, nearby stars.