His flight, 21, was delayed for hours.
答案：(C) however
語意轉折：「他的飛行原本應該只有15分鐘，然而卻延誤了好幾個小時」，however（然而）符合這個轉折；therefore（因此）、similarly（同樣地）、otherwise（否則）都不符合這個轉折語意。
however 你多半只在句首用過——However, he was late.——所以看到空格夾在 His flight 與 was 之間、兩邊各一個逗號，第一個念頭是「這個位置放不了 however」。再加上前一句結尾那個 so 讓因果的慣性還沒斷，手就滑向 therefore。答對率只有五成、鑑別度也只有 33，連高分群考生也大量失手，卡的就是這兩點：位置上的成見，加上被前一句的 so 帶著走。
四個選項都是轉承語，可見這一格考的是前後兩句之間的邏輯關係，那就把兩句擺在一起看。前一句是 His flight was supposed to last only 15 minutes, so there was no provision made for him to relieve himself in the capsule（他的飛行原訂只有十五分鐘，所以艙內沒有為他準備如廁的設施）；這一句是 His flight ___ was delayed for hours（他的飛行卻延誤了好幾個小時）。原本的預期與實際發生的事相反，這是轉折，however 正是做這件事的。位置本身其實是線索而不是障礙：therefore、however 這一類詞歸在「連接副詞」（linking adjunct）底下，可以出現在副詞能出現的任何位置（劍橋 English Grammar Today），夾在主詞與動詞之間、兩邊各加一個逗號，是它非常常見的擺法（同型：93 年第 29 題、101 年第 18 題、102 年第 17 題、97 年第 30 題、105 年第 16 題）。
therefore（因此）：同向的因果推論。前一句的 so 已經把因果講完了（原訂只飛十五分鐘，所以沒準備如廁設施），這一句要說的是計畫被打亂，方向相反。similarly（同樣地）：要求前後兩句是同一個方向的事，這裡不是。otherwise（否則）：需要前面先有一個「如果不那樣做」的前提，本句沒有。
be supposed to V（原本應該……）、be delayed for hours（延誤好幾個小時）、on the other hand（另一方面）、and yet（然而）、nevertheless（儘管如此）、otherwise（否則）。
The first American space toilet was Alan Shepard's space suit. His flight was supposed to last only 15 minutes, so there was no provision made for him to relieve himself in the capsule. His flight, 21, was delayed for hours. It was then decided that Alan Shepard should relieve himself in his space suit, and continue with the mission as scheduled.
22 problem urged NASA to develop the first space toilet, a modified diaper. Diapers are 23 used by astronauts when they are outside the space shuttle, wearing a spacesuit.
As the flights grew longer, a new set of two collection bags was devised, 24 for liquid wastes and the other for solid wastes. Once used, the bags are sealed and stored for the flight back to the earth, 25 they are disposed of.