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Verbal·Standard English Conventions·Form, Structure, and Sense
easy
Which choice completes the text so that it conforms to the conventions of Standard English?
The outer limit of the Sun’s magnetic influence is known as the heliopause. This invisible boundary, which        created by the solar wind (charged particles from the Sun) meeting interstellar space, has been crossed by only two spacecraft: Voyager 1 and Voyager 2.
A
is
B
are
C
were
D
have been