When We Determine the Age of the Universe Using the Hubble

When we determine the age of the universe using the Hubble Time, what important simplifying assumption goes into our calculations?

A) that the expansion of the universe has been happening at the same rate - neither speeding up or slowing down
B) that all the galaxies are moving away from us at exactly the same speed, no matter how far away they are
C) that the cosmological principle doesn't hold: the universe is not isotropic and homogeneous
D) that we have accounted correctly for the effects of dark energy
E) that the universe is actually contracting instead of expanding