Her son believed that when she ran out of money all she had to do was go to a bank machine, put in her ATM card and money would come out. 21 the child didn’t quite understand was that his mother had to have money in the bank in the first place to get money out of the machine later.
答案：(A) What
what 引導名詞子句作句子主詞，「這個孩子不太理解的是……」；When（何時）、Why（為什麼）、Where（哪裡）都不合。
本題大考中心未公布逐題答對率，以下不談數據，只談觀念。
看到空格出現在句首、後面接一長串，很容易把它當成前面第 19 題那種「回指前文」的情形而挑 That 之外的近似選項。這一格與第 19 題正好相反——這一次子句缺了東西。
先把句子的結構拉出來：___ the child didn’t quite understand was that his mother had to have money in the bank ...動詞是 was，所以空格帶出來的一整串（___ the child didn’t quite understand）是這個句子的主詞，是一個名詞子句。再數那個子句的成分：主詞是 the child、動詞是 didn’t understand，但 understand 是及物動詞，它的受詞不見了。既然缺一個受詞，就需要一個字進去補位，而且那個字還要能把整串變成名詞子句去當句子的主詞——同時做到這兩件事的只有 What（相當於 the thing which）。整句是「這孩子不太懂的是：媽媽得先在銀行裡有錢，之後才領得出來」。把這一題和第 19 題擺在一起，這條判準就完整了：第 19 題的 means 後面成分已滿，只需要一個單純的指示代名詞出現在主詞位置，用 That；本題的 understand 缺受詞，需要一個字進去補位，用 What。數成分不是為了刷掉 What，是為了決定「這裡需不需要一個字去補位」。（同判準：83 年第 27 題。）
When（何時）、Why（為什麼）、Where（哪裡）：三個都是副詞性的疑問詞，本身不占句子成分，補不了 understand 缺掉的受詞；語意上也不通——孩子不懂的不是時間、原因或地點，是「一件事」。
What sb doesn’t understand is …（某人不懂的是……）、run out of money（錢用光）、in the first place（一開始、首先）、a bank machine（提款機）、take money out（把錢領出來）、share an idea（分享一個想法）。
During a money management class, a mother shared her young child's idea about money. Her son believed that when she ran out of money all she had to do was go to a bank machine, put in her ATM card and money would come out. 21 the child didn't quite understand was that his mother had to have money in the bank in the first place to get money out of the machine later.
It may surprise us how much our children already know about money. They learn by watching what their parents do with money. This starts at a very young age. Kids watch their parents 22 things at stores, write checks, use credit cards or even make donations. Children also learn that there are often strong 23 connected with money. They learn how their parents feel about money by the tone of voice they use and the look on their faces.
24 , parents should ask themselves if they want their children to handle money the same way they do. The long-term 25 of teaching children good money habits is well worth the effort. Those who learn good money management skills are more likely to become adults who make sound financial decisions, avoid excessive debt, and reach their financial goals.