it can be shared by anyone__is close,是who還是whom?

題目：

it can be shared by anyone__is close,是who還是whom?
這是完型填空裡的一句話,橫線處應該是who還是whom?by後面應該是賓格,所以應該是whom,但爲什麼答案是who?
完型全文如下：
People all over the world celebrate Valentine’s DayHowever,the holiday 26 differently in different countries 27 each culture has its own Valentine’s Day customs For examplepeople in the United States and Japan both celebrate Valentine’s Day 28February 14But in Japanonly romantic partners come together,while in Americait can be shared by anyone 29 is closefriend or loverChocolate is the most preferred gift in the US 30 it is common in JapantooHowever,in the US other kinds of gifts are 3 1 givenand many people exchange cards The biggest 32 is that in Japanonly girls and women33 chocolates to boys and menbut in the US boys and girls will give cards or small gifts to all of 34 friendsAnd while American men and women both receive giftswomen usually get 35 expensive gifts than menThat’s why I would like to be a man in Japan but a woman in the US!

解答：

【考查】同位語
【難度】中
【題源】2012年廣州英語中考試卷
【解析】一個名詞(或其它形式)對另一個名詞或代詞進行修飾,限定或說明,這個名詞（或其它形式）就是同位語.本題空格中作爲anyone同位語,所以應該用who（同位語從句中who作主語,whom是賓語形式,不符題意）
【例句】Yesterday I met Tom,a friend of my brothers'.昨天我遇到了我弟弟的朋友湯姆.
（a friend of my brothers'是受詞Tom的同位語,指同一人）
【答案】who