.不知道应该选什么,还有为什么..8.The effectiveness of colonoscopy in detecting cancer _____ demonstrated time and again.(A)\x05are(B)\x05has been (C)\x05to be(D)\x05were

.不知道应该选什么,还有为什么..8.The effectiveness of colonoscopy in detecting cancer _____ demonstrated time and again.(A)\x05are(B)\x05has been (C)\x05to be(D)\x05were
.不知道应该选什么,还有为什么..
8.The effectiveness of colonoscopy in detecting cancer _____ demonstrated time and again.
(A)\x05are
(B)\x05has been
(C)\x05to be
(D)\x05were

.不知道应该选什么,还有为什么..8.The effectiveness of colonoscopy in detecting cancer _____ demonstrated time and again.(A)\x05are(B)\x05has been (C)\x05to be(D)\x05were
句子主语是effectiveness,所以谓语动词用单数,从选项来看,只能选B,
不知道是不是有单词打错,感觉不太通顺,
句意大概是:时间又一次证明了结肠镜检查在诊断癌症方面的效果

选B？？？

选B 结肠镜检查在发现癌症上的作用已经被一次有一次的证实了。