Unfortunately, shoes are not always readily available for people living in poverty, 21 shoes that are the right size.
答案：(D) let alone
let alone 用在否定句之後，意思是「更不用說」，表示連前面較容易的事都做不到，後面更難的事更不可能；except for（除了…之外）會使語意相反，provided with（被提供）句構不合，far from（遠非）語意不通。
let alone 照字面像「讓…獨自」，這樣拆就完全走偏。它是固定用語，意思是「更不用說」。使用有兩條鐵則：前面必須是否定的說法，而且 let alone 後面接的是「更難、更進一階」的那件事。
先看前後兩段的難易關係。前半 shoes are not always readily available for people living in poverty（貧困地區的人連鞋子都不見得有）是否定；後半 shoes that are the right size（合腳的鞋）要求更高。連鞋子都不一定有，更別說合腳的——遞進關係很清楚，兩條鐵則都成立。驗算法：把空格換成「更不用說」唸一遍，若前面那句不是否定的，或後面那件事反而比較容易，就代表選錯了。同義說法一起記：let alone ＝ much less ＝ not to mention ＝ to say nothing of，四個用法一致，可以輪流用。He couldn't boil water, let alone cook a meal for eight.（他連水都不會煮，更別說張羅八個人的一餐。）一句記完：He barely has time for breakfast these days, let alone for the homework he keeps putting off.（他這陣子連早餐都幾乎沒時間吃，更不用說那些一再拖延的功課。）〔情境取材自 101 學測作文題：朋友沉迷電玩〕
except for 是「除了…之外（其餘全部）」，屬於排除，會讀成「除了合腳的鞋以外其他都有」——與前半的否定正好相反。provided with 是「被提供」，是分詞形式，後面要接被提供的東西，且需要一個主詞來承接，句構上接不進來。far from 是「遠非、一點也不」，後面接形容詞或名詞表示強烈否定（far from perfect），放進來會變成「遠非合腳的鞋」，語意讀不通，而且它表達的是程度上的否定，不是遞進。
not…, let alone…（更不用說）、much less（更不用說）、not to mention（更別提）、to say nothing of（更別提）、readily available（隨手可得）、far from（遠非）。
Shoes are hugely important for protecting our feet, especially in places like Africa, where healthcare provision is limited. Unfortunately, shoes are not always readily available for people living in poverty, 21 shoes that are the right size. Almost as soon as a child receives shoes to wear, he/she is likely to have grown out of them. Then the child has to 22 with shoes that are too small.
The Shoe That Grows, created by a charity called Because International, changes all this. It allows children to 23 their shoes' size as their feet grow. The innovative footwear resembles a common sandal and is made of leather straps and rubber soles, a material similar to that used in tires. They come 24 two sizes, and can expand in three places. The straps on the heel and toe control the length of the shoe, 25 the two on either side allow for different widths. With this special design, the shoes can "grow" up to five sizes and last for at least five years.