Only by entering this quiet mental state 25 a climber really do his or her best.
答案：(A) can
【句型】Only by + V-ing 引導部分倒裝句時，主詞後面接助動詞 can + 原形動詞，表示「唯有透過……才能夠」；shall／must／should 都不符合這裡「才能夠」的語意（can）。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
四個選項都是助動詞，後面又都接得了原形 do，看起來一個也刷不掉。而這一句的語序還有點怪——助動詞跑到主詞 a climber 前面去了，很容易愣住。那個「怪」正是這一格的線索。
先看句子的形狀：Only by entering this quiet mental state ___ a climber really do his or her best.句首是 Only by ＋ 動名詞，而主要子句的助動詞跑到主詞 a climber 前面——這是倒裝。這是一組很常考的句型：否定或限制的副詞（片語）放到句首時，後面的主要子句要倒裝，助動詞提到主詞前面。這一家的成員包括 Not until …、No sooner … than …、Not only … but also …、Hardly … when …、Never、Seldom、Little，以及本題的 Only ＋ 副詞片語（Only by、Only then、Only after、Only when）。（Only 嚴格說是「限制」不是「否定」，但它與否定詞共用同一條倒裝規則，課本習慣放在一起教。）既然四個選項都是助動詞、形態上刷不掉，就用語意分：整段講的是攀登者唯有進入這種安靜的心理狀態，才「能夠」真正發揮實力——講的是能力，填 can。還原成不倒裝的說法就更清楚了：A climber can really do his or her best only by entering this quiet mental state.順便對照 95 年第 25 題，那一題考的是 Not until 與 No sooner，判準完全一樣——先看主要子句有沒有倒裝，沒有倒裝就直接刷掉那個選項，比慢慢想語意快得多。
must（必須）：講義務，變成「唯有進入這種狀態，攀登者才必須發揮實力」，語意不通。should（應該）：講建議或責任，同樣不是這一句要說的。shall：現代英語裡多用於徵詢意見（Shall we go?）或法律條文，語氣與這一句不合。
Only by V-ing ＋ 倒裝（唯有透過……才……）、Not until … ＋ 倒裝（直到……才……）、No sooner … than …（一……就……）、Not only … but also …（不但……而且……）、do one’s best（盡全力）、enter a state（進入某種狀態）。
Climbing the most difficult mountains requires a very special mental condition. 21 the climber is doing something very difficult, everything that he or she sees, hears, or 22 must come together to help in the work of climbing.
All of the parts of the body work together smoothly and quietly 23 each part feels what is happening in the other parts. There is no room for thinking about what time it is, or about how one looks. 24 thinking must focus on what is happening at that moment, and the mind and the body become one. Only by entering this quiet mental state 25 a climber really do his or her best.