【正确答案】

【答案解析】had fastened [解析] 本题考查的是if引导的条件状语从句中的虚拟语气。表示与过去事实相反的假设，用的句式为“if+主语+had done，主语+would/should/could/might+have done”。如：If you had come earlier, you might have met him. (如果你早点来，就能遇见他了。)
与现在事实相反时，从句是“动词过去式/were”，主句是“would/should/could/might+动词原形”。与将来事实相反时，从句是“动词过去式/were+to do”，主句为“would/should/could/might+动词原形”。如：If I saw him tomorrow, I should ask him about it. (如果明天见到他的话，我会问他这件事。)