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

easy

Mathematician Grigori Perelman, sometimes in conjunction with mathematicians Richard S. Hamilton and Shing-Tung Yau,        credited with proving the Poincaré conjecture. Having built on Hamilton’s previous work to solve the proof, Perelman has insisted that Hamilton receive credit. Yau later found and closed gaps in Perelman’s proof, persuading some mathematicians that he deserves credit as well.

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

A

is

B

have been

C

are

D

are being