—What' s happened to Jack? —When climbing the hill, he was knocked unconscious by an______rolling stone.
【正确答案】 B
【答案解析】untouched：未接触的；unexpected：想不到的，意外的，突然的；unfamiliar：不熟悉的；unbelievable：令人难以置信的。根据句意可知，选B。