Given the equation representing a phase change at equilibrium:

h2o (s) < --> h2o (l)
which statement describes this equilibrium?
a. the h2o (s) melts fadter than the h2o (l) freezes
b. the h2o (l) freezes faster than the h2o melts
c. the mass of h2o (s) must equal the mass of h2o (l)
d. the mass of h2o (l) and the mass of h20 (s) each remain constant.