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。
答對率 24%。
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：前面必須是否定句，而且 let alone 後面接的是「更難、更進一階」的那件事。
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）顯然要求更高。連鞋都不一定有，更別說合腳的——遞進關係很清楚。
let alone ＝ much less ＝ not to mention ＝ to say nothing of，四個都是「更不用說」，用法一致。He couldn't boil water, let alone cook a meal for eight. 換成任一個都成立，寫作時可以輪流用。
except for 會變成「除了合腳的鞋以外都有」，剛好相反；provided with（被提供）詞性不合；far from 後面接形容詞或名詞表強烈否定。
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）。
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.