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Verbal·Standard English Conventions·Form, Structure, and Sense

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The outer edge of the Sun’s magnetic reach is called the heliopause. This invisible border, which        created by the solar wind (charged particles from the Sun) colliding with interstellar space, has been crossed by only two spacecraft: Voyager 1 and Voyager 2.

Which choice completes the text so that it conforms to the conventions of Standard English?

A

is

B

were

C

are

D

have been