Mashaal Masha
In 1990, California native and researcher Ellen Ochoa left her position as chief of the Intelligent Systems Technology Branch at a NASA research center ________ the space agency's astronaut training program.

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

to join

B: 

is joining

C: 

joined

D: 

joins

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