If simple sugars such as ribose and glycolaldehyde ______ blank Earth from elsewhere and survived impact-a

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 indie 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 indie 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 indie that the simple sugars reached Earth before reacting with other molecules on the planet.