The alga species Chlorella vulgaris is very eﬃcient at making oxygen. For this reason, scientists are currently exploring ways
to use this species in space. C. vulgaris might be used, for example, to build future biological air exchange systems that
______ oxygen for astronauts.
are producing
produced
produce
have produced
Choice C is the best answer. The convention being tested is the use of verbs to express tense. Simple present tense verbs
can be used to describe actions that tend to occur, including in a hypothetical or future scenario. In this case, the simple
present tense verb “produce” indicates what the air exchange systems might be able to do in the future (produce oxygen for
astronauts).
Choice A is incorrect. The present progressive tense verb “are producing” suggests that the oxygen is currently being
produced, not that it might be produced in the future. Choice B is incorrect because the past tense verb “produced” suggests
that the oxygen was produced in the past, not that it might be produced in the future. Choice D is incorrect because the
present perfect tense verb “have produced” suggests that the oxygen has been produced from a point in the past up to the
present, not that it might be produced in the future.