Which statement is true about gravitational forces?
a.
only masses of equal size exert a gravitational force on each other.
b.
every mass exerts a gravitational force on every other mass.
c.
only large masses exert a gravitational force on small masses.
d.
only moving masses exert a gravitational force on stationary masses.

Crinoids, also known as sea lilies, appear to be sea plants but are not plants at all. crinoids are an echinoderm species which uses anchoring structures called holdfasts to attach themselves to the sea floor. crinoids were most abundant in pennsylvania during the mississippian period, which ranged from 375 to 320 million years ago. fossils of the crinoids shows that during most of this time pennsylvania was that provided favorable conditions for crinoid growth. a) glacial terrain b) a dry and sandy plain c) mostly mountain ranges d) covered by warm, shallow seas