If simple sugars such as ribose and glycolaldehyde ______ Earth from elsewhere and survived impact—a possibility
astrophysicist Nicolle Zellner outlined in a 2020 study—the sugars could have reacted with other molecules that were already
present on the planet to form the nucleotides that are the structural components of RNA and DNA.
reach
had reached
will reach
are reaching
Choice B is the best answer. The convention being tested is the use of verbs to express tense. In this choice, the past perfect
verb “had reached” is used correctly to describe a past action that was completed before another action in the past.
Speciﬁcally, in this hypothetical scenario about the origins of RNA and DNA on Earth, the simple sugars had to have reached
Earth before they could react with other molecules on the planet.
Choice A is incorrect because the present tense verb “reach” doesn’t indicate that the simple sugars reached Earth before
reacting with other molecules on the planet. Choice C is incorrect because the future tense verb “will reach” doesn’t indicate
that the simple sugars reached Earth before reacting with other molecules on the planet. Choice D is incorrect because the
present progressive verb “are reaching” doesn’t indicate that the simple sugars reached Earth before reacting with other
molecules on the planet.