The straps on the heel and toe control the length of the shoe, 25 the two on either side allow for different widths.
答案：(B) while
前後兩個子句是並列對比關係（控制長度 vs 控制寬度），while（而/然而）連接兩個對等子句表達這種對比；whether（是否）、with（不能這樣連接兩個完整子句）、for（因為，這裡是並列對比不是因果）都不合。
while 你腦中第一個意思是「當……的時候」，於是你去找兩件同時發生的事，結果發現「綁帶控制長度」和「兩側調整寬度」根本不是在講時間，就卡住了。while 當連接詞其實有兩個意思：一個表時間，另一個表對比，可以翻成「而」「然而」（劍橋 English Grammar Today 把 while 收在「表對比的連接詞」這一類底下）。
先數子句。空格前是一個完整子句：The straps on the heel and toe control the length of the shoe；空格後也是一個完整子句：the two on either side allow for different widths。兩個完整子句要接在一起，中間一定要有連接詞。再看關係：一個管長度、一個管寬度，是同一件事情的兩面對照，不是因果也不是條件，所以選表對比的 while，換成 whereas 讀也完全通。這一題鑑別度偏低，代表 with 這個選項連高分群考生也會踩。原因是 with 讀起來很順口（「兩側的則……」），但 with 是介系詞，後面只能接名詞或動名詞，不能接 the two ... allow ... 這種完整子句。這是詞性能不能接子句的問題，不是語意問題——遇到兩個完整子句時，先把不能接子句的選項刪掉，範圍馬上縮小。
whether（是否）引導的是名詞子句，代進去無法表達對比。with 如前所述不能接完整子句。for（因為）表原因，但控制長度並不是控制寬度的原因，前後是並列不是因果。表對比的 while 在 102 年第 23 格、107 年第 21 格都考過，107 年那題還放在句首寫成大寫的 While。三題的解法完全一樣：先看前後兩句是同向還是反向，再挑連接詞。
while（然而、而，表對比；也可表「當……的時候」）、whereas（然而，表對比）、on the other hand（另一方面）、allow for（容許有、考慮到）、on either side（兩側）、control the length（控制長度）。
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.