摘要：31．―Do you know ? ―Tomorrow morning．A．when the visitors will come here B．when will the visitors come hereC．how the visitors will come here D．how will the visitors come here
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Of all the inventions, what do you think is ________ invention in the 21st century．
- A.the most helpful
- B.more helpful
- C.helpful
- D.more and more helpful