把两个句子变为一个时,连接词的问题（英语）
the temple is the oldest in Egypt.We visited it.
变成一个句子是
the temple we visited is the oldest in Egypt.
The power stations is very dirty.It burns coal.
变成一个句子是
The power stations which burns coal is very dirty.
为什么第一个句子不用连接词,而第二个句子要用which?