Which choice completes the text so that it conforms to the conventions of Standard English? In 1990, California native and researcher Ellen Ochoa left her position as chief of the Intelligent Systems Technology Branch at a NASA research center ______blank the space agency's astronaut training program.

Choice A is the best answer. The convention being tested is the use of nonfinite verb forms in a sentence. The nonfinite to-infinitive verb "to join" is correctly used to form a subordinate clause that expresses why Ochoa left her position (to join the training program).Choice B is incorrect because the finite verb "is joining" can't be used in this way to indicate Ochoa's action of joining the training program. A conjunction such as "and" would be needed to coordinate "is joining" with the previous finite verb, "left." Choice C is incorrect because the finite verb "joined" can't be used in this way to indicate Ochoa's action of joining the training program. A conjunction such as "and" would be needed to coordinate "joined" with the previous finite verb, "left." Choice D is incorrect because the finite verb "joins" can't be used in this way to indicate Ochoa's action of joining the training program. A conjunction such as "and" would be needed to coordinate "joins" with the previous finite verb, "left."