I am not surprised that the Murphys are not here now. They ________ been delayed at home because the streets are so crowded today.
句意： 莫斐一家现在不在， 我并不感到奇怪。 他们一定是被耽搁在家里了， 因为今天街上太拥挤了。 must have done 是对过去发生的动作最有把握的猜测， 意思是“一定”， 符合句子逻辑。 故正确答案为C。