The term “lucid” means clear; lucid dreamers know that they are dreaming and 26 they are dreaming of.
答案：(A) what
what 引導名詞子句作 dreaming of 的受詞，「知道自己在做夢，也知道自己夢到了『什麼』」；why（為什麼）、when（何時）、which（需要明確先行詞）都不合。
這一格答對率 77%，不算難，但錯的人多半是被 and 前面那個 that 帶著走，覺得後面也應該接 that。
先看 and 連接的是什麼：know 帶了兩個受詞，第一個是 that they are dreaming（他們正在做夢），and 之後接的第二個受詞就是空格帶出的名詞子句。再看子句的尾巴：they are dreaming of ______，of 是介系詞，後面缺一個受詞。所以要填的字必須同時做兩件事——把子句接上去，而且自己補上那個缺掉的受詞。能一人身兼兩職的就是 what，因為 what 本身等於 the thing(s) that（……的那個東西），先行詞已經包在自己裡面了。還原一次就很清楚：they know what they are dreaming of ＝ they know the thing that they are dreaming of（他們知道自己夢到的是什麼）。給你一個好用的判斷訣竅：句尾的介系詞後面缺受詞，就用 what，不用 that。
why（為什麼）、when（何時）代進去，dreaming of 後面依然缺受詞，句子補不完整。which 需要有明確的先行詞或限定的選擇範圍（例如 which one），這句兩者都沒有。
what ＝ the thing(s) that（……的事物）、know what（知道……是什麼）、dream of／about（夢見……）、be immersed in（沉浸在……之中）、lucid dream（清醒夢）、daydream（白日夢；做白日夢）、waking thoughts（清醒時的思緒）。
You must have had this kind of experience: While in the middle of a normal dream, you suddenly realize that you are dreaming. This kind of dream is called a "lucid dream." The term "lucid" means clear; lucid dreamers know that they are dreaming and 26 they are dreaming of.
It is different from daydreaming. When a person is having a lucid dream, the person's body is 27 ; when a person daydreams, his/her body is awake. Thus, daydreams are really just waking thoughts. In lucid dreams, however, we are completely immersed in the dream world.
Yet, lucid dreaming is 28 just having a clear dream. It is your chance to play around with the extraordinary abilities buried in unused parts of your brain. 29 , it is a way for you to put the deepest areas of your brain to good use while you're sleeping. You can be an everyday Jane Doe or John Smith while awake but a superhero while sleeping, 30 who you are in real life. All the obstacles of reality can be set aside and you are able to accomplish tasks that you could never manage in waking reality.