Can you tell me the right reason ( ) you was absent for ten

Can you tell me the right reason ( ) you was absent for ten days?
A.that B.why C.for D.because
正确答案是A,可我觉得是B啊,怎么理解呢?
The reason why he was absent is that he was caught in the traffic jam.根据这个句子应该选B的不是?

这里是考察同位语从句的引导词, 先行词是reason, 所以用that从句做同位语.
同位语部分是个句子,就是同位语从句,这种用法比较"固定",把关键的几个词背下来.
下面这个材料供参考.

一、在复合句中用作同位语的从句叫同位语从句.它一般跟在某些名词后面,用以说明该名词表示的具体内容.如：
I heard the news that our team had won．我听到了我们队获胜的消息.
I had no idea that you were here．我不知道你在这里.
二、可以跟同位语从句的名词通常有new,idea,fact,promise,question,doubt,thought,hope,message,suggestion,word（消息）,possibility等.如：
I’ve come from Mr wang with a message that he won’t be able to see you this afternoon． 我从王先生那里来,他让我告诉你他今天下午不能来看你了.
三、英语中引导同位语从句的词通有连词 that,whether,连接副词 how,when,where等.（注：if,which 不能引导同位语从句.）如：
l have no idea When he will be back．我不知道他什么时候回来.
He must answer the question whether he agrees to if or not.
他必须回答他是否同意这样一个问题.
四、有时同位语从句可以不紧跟在说明的名词后面,而被别的词隔开. 如：
Several years later,word came that Napoleon himself was coming to inspect them．
几年以后,有消息传来说拿破仑要亲自视 察他们.
The thought came to him that maybe the enemy had fled the city．
他突然想起可能敌人已经逃出城了.
五、同位语从匈与定语从句的区别.
1、同位语从句与前面的名词是同位关系,即说明它前面名词的内容；而定语从句与前面的名词是修饰与被修饰关系,即限定它前面的名词范围,或补充一些情况.如：
The news that l hove passed the exam is true．我通过了考试这一消息是真的.
（同位语从句,即从句所表达的意思就是前面名词的内容.）
The news that he told me just now is true．他刚才告诉我的消息是真的.
（定语从句,从句对前面名词起修饰限制作用,即“他告诉我的”那个消息,而不是别的消息.）
2、引导同位语从句的that是连词,在从句中不充当任何成份,而引导定语从句的that是关系代词,除起连接作用外,还在从句中充当主语、宾语或表语等.如：
The idea that computers can recognize human voices surprises many people.
计算机能够识别人的声音的想法使许多人感到惊奇.（that在从句中不充当任何成份.）
The idea that he gave surprises many people．他提出的观点令许多人感到吃惊.
（that在从句中作gave的宾语.）