It was twelve o'clock they finished the work. [ ] A．sinceB．whichC．thatD．when

考生可能受强调句型的思维定势影响而选C。若选C，将其还原，则变成：They finished the work twelve o'clock．显然twelve o'clock前少了介词at，故错。因此可判定该题为定语从句，关系副词when在从句中作状语，it是指代时间的非人称代词，不属于强调句型中的it。