The only language----is easy to learn is the mother tongue.A which B that C / D it 请说明下原因

The only language----is easy to learn is the mother tongue.A which B that C / D it 请说明下原因
The only language----is easy to learn is the mother tongue.A which B that C / D it 请说明下原因

The only language----is easy to learn is the mother tongue.A which B that C / D it 请说明下原因
B that
有only修饰先行词,用that,因为这里关系代词做主语,所以不省略

B 当先行词是
关系代词that 用法
that常用作关系代词，可指代某人，也可指代某物，指物时有的时候可用which替换that，指人时可用who替换。但在下列情况下，一般用that。
一、that指代某物事时
1.先行词为all, few, little, much, the one, something, anything, everything, noth...

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B 当先行词是
关系代词that 用法
that常用作关系代词，可指代某人，也可指代某物，指物时有的时候可用which替换that，指人时可用who替换。但在下列情况下，一般用that。
一、that指代某物事时
1.先行词为all, few, little, much, the one, something, anything, everything, nothing等时。如：
（1）We'll do all that we can to protect the endangered plants and animals.
我们将尽我们的最大努力来保护那些濒危的动植物。
(2) There is much that I wan to tell you.
我有很多想要告诉你的话。
(3) Is there anything that I can do for you?
有什么我可以帮你的吗？
2. 先行词被all, every, no, some, any, (a)few, (a)little, much等限定词修饰时。如：
(4) You can borrow any book that you want to read in our school library.
在我们图书馆，你可以借任何你想读的书。
3. 先行词被序数词或形容词最高级修饰时。如：
（5）This is the most beautiful city that I've ever seen.
这是我见过的最漂亮的城市。
4. 先行词被the only, the very, the right, the last 等修饰时。如：
（6）This is the very factory that they visited last summer holiday.
这恰好是他们去年参观的那个工厂。
（7）This is the only painting in this style that we have.
这种风格的画我们仅有一幅。
5. 先行词是并列结构，既有人又有物时。如：
（8）He talked happily of the writer and his books that interested him.
6. 先行词前有the same 修饰，表示和先行词是同一物时。如：
（9）This is the same purse that I lost yesterday.
这就是我昨天丢的那个钱夹。
注：如果表示的是与先行词同一类或相似的某物，则用the same…as….如：
（10）This is the same purse as I lost yesterday.
这个钱夹和我昨天丢的那个一样。
7. 先行词为数词时。
（11）Look at the books on the bookshelf. You can see the two that you bought me for my birthday.
瞧书架上那些书。你可以看到我过生日时你买的那两本。
8. 如果which引导的定语从句中又含有一个定语从句，为避免重复，第二个定语从句用that。如：
（12）They built a factory which produced things that had never been seen.
他们建了一工厂，生产以前从未见过的东西。
9. 以which作主语开头的特殊问句，为了避免重复，定语从句用that。如。
（13）Which is the bus that you will take?
你要乘的是哪一班车？
10. 关系代词在定语从句中作表语时，而且通常省略。如：
（14）My hometown is no longer the place that it used to be.
我的家乡再也不是以前那个样子了。
11. 关系代词在there be 句型中作主语时,而且常可以省略。如：
（15）This is the fastest train (that) there has ever been.
这是有史以来最快的列车。
二、that 指代某人时。
1. 泛指某人时。如：
（16）He is a man that is never at a loss.
他是一个从未一筹莫展的人。
2. 主句是以作主语的who开头的特殊问句，为了避免重复时。如：
（17）Who is the person that is talking with our headmaster?
和我们校长说话的那人是谁？
3. 先行词前有the same时。如：
（18）This is the same man that gave us a talk last year.
这和去年给我们作报告的是同一人。
4. 关系代词在定语从句中作表语时。如：
（19）He has changed. He was not the man (that) he was 10 years ago.
他变了。他再也不是10年前的他了。
另外，that也可用作关系副词，表示时间、地点、原因和方式，在口语中可以用来代替when, where, why 引导的定语从句。在以下名词day, year, time, moment, reason, place等作先行词时，可用that作关系副词引导定语从句。如：
（20）I'll never forget the day (that) I joined the League.
我永远不会忘记我入团的那一天。
（21）Is this the reason (that) they were late for the meeting?
这就是他们开会迟到的原因吗？
（22）We want to find a place (that) we can have a picnic.
我们想找一个我们能野餐的地方。
（23）This is the first time that I have been abroad for traveling.
这是我第一次到国外去旅游。
（注：先行词是time,前面有序数词或last修饰时，常用that引导定语从句或者省略。）
当先行词为表示方式的词the way时，可用that引导定语从句, that常可以省略。
（24）I don't the way you speak to her.
我不喜欢你和她说话那种方式。

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B 当先行词被 The only 等修饰时必须用that