Mashaal Masha
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.

Which choice completes the text so that it conforms to the conventions of Standard English?
Difficulty: Easy
A: 

are producing

B: 

produced

C: 

produce

D: 

have produced

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