大学英语：The weather of Nanjing is hotter than that of Qingdao

　1) 南京的天气比青岛的天气热。（The weather of Nanjing is hotter than that of Qingdao.）

　　2) 黄金的价格比白银的价格要高。（The price of gold is higher than that of silver.）

8. Other states have a sales tax, which is a percentage charged to any item which you buy in that state.

　　句中前一个which 引导的是一个非限定性定语从句，修饰sales tax。非限定性定语从句通常在句中起进一步说明作用，既可修饰先行词也可修饰整个句子。如果省去，原句意义不受影响。非限定性定语从句有逗号与主句隔开；关系代词不可用that，而用who, whom, whose来修饰人，用which来修饰物，并且不能省略。

请翻译下面的句子：

　　1) The sun warms the earth, which makes it possible for plants and animals to live.（太阳温暖了大地，这才使动植物有可能生长。）

　　2) He failed the exam, which made him very disappointed.（他考试没及格，这使他非常失望。）

　　3) He has a son, who is doing his PH.D in the United States.（他有一个儿子，在美国读博士学位。）

句中后一个which引导的是一个限定性定语从句，修饰any item。但是传统语法中通常有这样的规定：当先行词是all，everything，something，nothing等不定代词，或被first，only，few，much，some，any，no以及形容词最高级等词修饰时，应该用关系代词that，不用which。例如：I am interested in all that you told me.