関係代名詞、先行詞の決定、文脈

"He guessed that those marks were made by the blades attached to the smaller piece of the fan disk, which was still missing," Flight 232, Laurence Gonzales, W. W. Norton & Company 

ユナイテッド航空232便の不時着事故。文法的にはwhichはthe fan diskにも係り得るが、現場でfan diskは発見されているという文脈から、whichはthe smaller pieceに係ると分かる。