Starting from April 2017, the pills were placed in 40 bird feeders 20 throughout the city.
答案：(B) installed
過去分詞 installed 作後位形容詞修飾 feeders，表示「安裝在城市各處的鳥類餵食器」；installing（現在分詞，主動語意方向不合，應該是餵食器「被安裝」）、to install（不定詞）、having installed（完成分詞，不合這個被動語境）都不合。
看到 40 bird feeders ___ throughout the city，你會想「這裡缺一個動詞」，於是挑了 to install 或 installing。但這句話的主要動詞已經有了——the pills were placed in 40 bird feeders。空格要填的不是主要動詞，而是修飾 feeders 的形容詞成分。
還原成完整的關係子句最清楚：40 bird feeders that were installed throughout the city（安裝在城市各處的 40 個餵食器）。把 that were 拿掉，剩下的就是過去分詞 installed，這叫簡化關係子句——你在 the book written by him（他寫的書）、the money stolen yesterday（昨天被偷的錢）都見過同一個結構。要決定用哪個分詞，只要問一句：餵食器是「安裝別人」還是「被安裝」？ 被安裝，所以用過去分詞。
installing 是現在分詞，表主動，會變成「餵食器在安裝東西」；to install 是不定詞，表目的或未來，語意變成「為了安裝而準備的餵食器」；having installed 是完成分詞的主動形式，同樣把餵食器當成動作的執行者。
install（安裝）、installation（安裝、裝置）、be placed in（被放置在……）、throughout the city（遍及全市）、the book written by sb（某人寫的書）、the money stolen（被偷的錢）。
Street pigeons are a common sight in many European cities. In fact, a visit to a major European city just would not be the same 16 encountering them: bathing in fountains, perching on historic buildings, and flocking in public squares. In Barcelona, however, the birds are now threatening to 17 the city's historic center, pooping on monuments, buildings, and even tourists.
The city council originally proposed 18 the pigeon population by gathering the birds and shooting them. But that idea led to strong protests from numerous animal rights groups. After heated debates and discussions, the government was finally persuaded to consider another 19 effective method: giving the pigeons birth control.
In 2016, the city council decided to put its 85,000 pigeon population on a birth control pill to block the formation of eggs in birds. Starting from April 2017, the pills were placed in 40 bird feeders 20 throughout the city. It was predicted that there would be a decrease of 20 percent in the pigeon population in the first year and between 70 and 80 percent in four or five years.