38______ you wish to cancel the airplane ticket one month before its flight, please submit your application form with the ticket and receipt to customer service.
(A) Could
(B) Might
(C) Would
(D) Should

英文的條件句有三種情況可以省略 if，當條件句的動詞部分含有，可省略 if 使句子變成倒裝句。

1. were (與現在事實相反)

2.had (與過去事實相反)  

3.should (與未來事實相反 — 即假設的情況不太可能發生)

例如：

• If I were you, I wouldn't do that.
  = Were I you, I wouldn't do that. (如果我是你，就不會那麼做了)
• If he had taken my advice, he might not have made such a bad mistake.
  = Had he taken my advice, he might not have made such a bad mistake. (要是他聽我的勸告，就不會犯這樣嚴重的錯誤了)
• If I should have a chance to try it, I would do it in another way.
  = Should I have a chance to try it, I would do it in another way. (要是我有機會試一試，我會用另一種方法去做) — Should 係表示有機會一試的可能性微乎其微。
• 本題原句為：If you should wish to cancel the airplane ticket one month before its flight, please submit your application form with the ticket and receipt to customer service. （表示未來如果有要一個月前取消機票的話）
• 省略if 並將 should 移至句首 → Should you wish to cancel the airplane ticket one month before its flight, please submit your application form with the ticket and receipt to customer service.