Planetary scientist Briony Horgan and her colleagues have determined that as much as 25 percent of the sand on Mars is
composed of impact spherules. These spherical bits of glass form when asteroids collide with the planet, ejecting bits of
molten rock into the atmosphere that, after cooling and solidifying into glass, ______ back onto Mars’s surface.
to rain
raining
having rained
rain
Choice D is the best answer. "That…[rain] back onto Mars’s surface" is a relative clause that describes the "bits of molten
rock." Forming the clause requires a conjugated, ﬁnite verb, and this is the only choice that provides that.
Choice A is incorrect. "To rain" is an inﬁnitive and can’t serve as the main verb of a clause. A conjugated verb is needed here
to form the main verb of the relative clause "that…[rain] back onto Mars’s surface," which describes the "bits of molten rock."
Choice B is incorrect. "Raining" is a present participle and, on its own, can’t serve as the main verb of a clause. A conjugated
verb is needed here to form the main verb of the relative clause "that…[rain] back onto Mars’s surface," which describes the
"bits of molten rock." Choice C is incorrect. "Having rained" is a perfect participle and can’t serve as the main verb of a
clause. A conjugated verb is needed here to form the main verb of the relative clause "that…[rain] back onto Mars’s surface,"
which describes the "bits of molten rock."