they are nearly of an age these doors are of a size 为什么要用一个o

they are nearly of an age these doors are of a size 为什么要用一个of?

朋友,看了以下解释,相信你会悟到这个用法的.
一、“be of+抽象名词”,这时的of短语相当于这个名词的同根形容词,作为表语表示主语的性质或特征.of的意思是“具有、具备”等,常用于该结构的名词常见的有
of use=useful；of nouse=useless；of importance=important等.如：
The matter is of great importance.=The matter is very important.
在形容词前常用very来修饰,在“of+抽象名词”结构中,名词前常用great,much等来修饰.再如It is of great importance for college students to master a foreign language.
=It is very important...对大学生来说,掌握一门外语是很重要的.
二、“be of+集合名词或其它类型的名词”,相当于belong to或have.of表示归属关系或“有”的意思.如：The army and the people are of one family.
=The army and the people belong to one family.军民是一家.
三、“be of+物质名词”表示主语是“用某种材料构成或制作的”,相当于be made of或be built of.如：Our building was of bricks.=Our building was built of bricks.
我们的楼房是砖建的.
四、be of+(表示种类、颜色、年龄、形状、价格等）名词,多用来表示主语的特征.这类结构的主语可以是人,也可以是物,其中的of可以省略.常用于该结构的名词有：kind,age,colour,size,height,shape,type,way,price等.如：
Several of the stamps were of the unusual kind.其中几张邮票是不同种类的.
The two boys are of the same age.这两个男孩年龄相同.
She is of a different way of thinking.她的思维方式与众不同.
These two kinds of article are of the same price.这两种物品价格相同.
五、“be of+名词”也可以表示“来源、组成”.如：
He was of a poor peasant family.他出身贫农家庭.
The committee is of seven people.该委员会由7人组成.
六、“be of+形容词的最高级”相当于one of.如：
Mr.Wang is of the best teachers in our school.
=Mr Wang is one of the best teachers in our school.
王老师是我们学校里最好的老师之一.