What is the primary practical difficulty that limits the use of Hubble's law for measuring distances?
A) Redshifts of galaxies are difficult to measure.
B) The recession velocities of distant galaxies are so great that they are hard to measure.
C) We do not know Hubble's constant very accurately yet.
D) Hubble's law is only useful theoretically; it is difficult to use in practice.
E) The motion of Earth relative to the Milky Way is difficult to account for.
Question 2
What defines the habitable zone around a star?
A) the region around a star where rocky planets form
B) the region around a star where humans can survive
C) the region around a star where liquid water can potentially exist on planetary surfaces
D) the region around a star where the ultraviolet radiation does not destroy organisms on a planetary surface
E) the region around a star where life exists
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