【问】 “What did you buy at the market?”
 “A quart of milk，two pounds of steak，and four____. ”
 A. corn’s ears D. ears of corn
 C. corn ears D. ears of corns(TOEFL，71.3)
 答案为B(ears of corn—玉米穗)。但是，我怎么也弄不明白为什么不能选A(corn’sears)。我们不是可以说cow’s milk吗?
 【答】 不可否认，'s结构(N₁'s+N₂)与of结构(N₂+of+N₁)常常表达相同的意思。例如：
 some people's opinions=opinions of some people
 the city's population=the population of the city
 但是切不要认为凡是可用’s结构的地方就可以换用of结构，或者凡可以用of结构的地方就可以换用’s结构。实际上，它们两者的使用范围都是有限制的。其中一个很重要的限制就是属格名词N₁’s本身的词汇因素。这就是说，有些名词可以采用属格形式，而有些名词则不能采用属格形式，而必须采用of短语形式(N₂+of+N₁)。究竟哪些名词可采用属格形式呢?初学者可记住这样一个最简单的说法：动物名词和集合名词可采用属格形式，无生命名词则应采用of结构或名词+名词结构，但是，地理名词、地点名词、时间名词和运输工具例外。下面是关于这一说法的例证：
 1. the boy's age(这个男孩的年龄)
 2. the horse's tail(马尾)
 3. the committee's decision(委员会的决定)
 4. four ears of corn (4个玉米穗)
 5. the roof of the house(房顶)
 6. the United States'attitude(美国的态度)
 7. the city's atmosphere(城市的气氛)
 8. today's paper(今天的报纸)
 9. the plane's engines(飞机发动机)
 请做下面的英语试题
 1. “I've invited Bill over tonight.” “Then you'd better turn on_________. ”
 A. the porch's light B. the light porch
 C. the light's porch D. the porch light(TOEFL，74. 6)
 2. “What does Roger Brown do for a living? ”“ He's one of the most successful________inthe city. ”
 A. newspaper reporter B. newspaper's reporters
 C. newspaper' s reporter D. newspaper reporters(TOEFL，74.9)
 3. “He said that he forgot both of the_________. ”
 A. rooms number B. room number
 C. room's number D. room numbers(TOEFL，71.10;75.9)
 4. What do you call juice that comes from tomatoes? You call it______.
 A. tomatoes juice B. juice of tomato
 C. tomato juice D. tomato's juice(TOEFL 教程)
 5. It took_______ before the boat began to float better.
 A. an hour work B. an hour's work
 C. the work an hour D. a work hour(Nelson，450D)