In knot theory (the mathematical study of curved, closed loops), knots are characterized by their crossing numbers—that is, the number of times the knotted thread crosses over itself. The trefoil knot and the figure-eight knot, each with a crossing number below five, __________ among the simplest possible knots with the fewest number of crossings.
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Verbal·Standard English Conventions·Form, Structure, and Sense
easyWhich choice completes the text so that it conforms to the conventions of Standard English?
A
are
B
has been
C
is
D
was