网友的问题，涉及关系代词which 的指代范围。

which 引导非限制性定语从句时，有一个特殊的用法，就是：它除了指代主句中的某个名词或者整个主句的内容外，还可以指代主句中的某个名词词组、不定式、形容词或形容词短语等。例如：

The man was unable to earn enough money, which allowed him to support his family.

那人无法赚到足够的钱，只有足够的钱才能让他得以养活家人。

which 指代enough money，不是单个的money。更不是整个主句，因为整个主句的意思是“无法赚到足够的钱”，既然赚不到钱，怎么养活家人？

Chinese started to use chopsticks a long time ago, which gradually became a part of Chinese life.

中国人很早就开始使用筷子，这（使用筷子一事）逐渐成为中国人生活的一部分。

下面，把which 换成不定式to use chopsticks 看看是否成立：

To use chopsticks gradually became a part of Chinese life.

使用筷子逐渐成为中国人生活的一部分。

（动词不定式作主语，完全成立。）

此外，became 没错，指过去已发生。

又如：

1. He was not sick, which some of the other passengers were.

他没有生病，但有些其他旅客却病了。

（which 指代形容词sick）

2. She is always careless, which we should not be.

她总是马虎大意，我们可不应该这样。

（which 指代形容词careless）

3. The two policemen were completely trusted, which in fact they were.

那两个警察完全受到信任，事实上，也真是如此。

（which 指代形容词短语completely trusted）

4. She was very patient towards the children, which her husband seldom was.  

她对孩子们很耐心，她丈夫却很少这样。 

（which 指代形容词短语patient towards the children）

Chinese started to use chopsticks a long time ago, which gradually became a part of Chinese life.

参考译文：中国人很久之前就开始使用筷子，筷子逐渐成为中国人生活的一部分，

解答如下

(1)which指代的是“chopsticks”

(2)用has become更恰当。