With the help of high technology, more and more new substances ______ in the past years. A、 discovered B、 have discovered C、 had been discovered D、 have been discovered

With the help of high technology, more and more new substances ______ in the past years.
A、 discovered
B、 have discovered
C、 had been discovered
D、 have been discovered

出自：石家庄铁道大学-大学英语3

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