【题目】They left no stone ___________ in their desire to achieve their best.
A.turned
B.taken
C.unturned
D.undertaken

【答案】C
【解析】考查固定短语。句意：他们为了取得最好的成绩而竭尽全力。leave no stone unturned“千方百计；竭尽全力”是固定短语。故选C。