The researchers ....................... hard to develop a perfect new model ...

★ TOEIC Grammar and Vocabulary Review

The researchers ………………….. hard to develop a perfect new model over the last two years.

A. work

B. worked

C. are working

D. have been working

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Sunset Co. ………………… another branch office in Floria in the near future.

A. opened

B. had opened

C. will be opened

D. is going to open