Mary's friend/a friend of Mary's★
【问】 
that she was a friend of 
，she left 
her name. (TOEFL，75.11)
我已知道正确的答案是将Mary(B)改为Mary’s。但我不知道能否将a friend of Mary’s改为Mary’s friend。
【答】 在上面的句子里不能将a friend of Mary’s改为Mary’s friend。为什么呢?因为Mary’s friend是表示确定指，而a friend of Mary’s是表示不确定指，即不肯定、不熟悉。在上面的试题里恰恰需要一个不确定指的名词。类例：
1. It is being repaired by a friend of my father's. (NCE，Book 2，L. 10)
它正在由我父亲的一位朋友进行修理。
2.
3.
(参见double genitive)
赵振才 编著.英语常见问题解答大词典.哈尔滨：黑龙江人民出版社.1998.第452-453页.
- at any minute
- at a rate of/at the rate of
- at a time/at one time
- at a time/at one time
- at a time/at the time
- at birth/at a birth
- at Christmas/ Easter
- at Easter
- a ten-dollar pen/a ten-dollars pen(?)
- at home
- at/in the east
- at/in the playground
- at last/finally
- at no time/in no time
- at one time
- at/on the weekend
- at play/at the play
- at sea
- at sea/at the sea/on the sea/in the sea
- at table/at the table
- attain/attain to
- attain/obtain
- attempt
- attend at/on/to
- attention/attentions