In Barcelona, however, the birds are now threatening to 17 the city’s historic center, pooping on monuments, buildings, and even tourists.
答案：(C) take over
take over 是「接管、占據」，本句指鴿子正威脅著要占據巴塞隆納的歷史城區；set up（設立）、hang on（稍等、堅持）、break down（故障、破裂）語意都不合。
答對率 31%。
take over 多數人在商業新聞裡看過——「接管公司、併購」，於是很難把它跟鴿子聯想在一起。其實這個片語的核心是「取得控制權、把地盤接手過去」，主詞不必是人或公司；動物、機器、甚至情緒都可以 take over。
the birds are now threatening to ___ the city's historic center（這些鳥如今正威脅著要…這座城市的歷史城區），後面緊接 pooping on monuments, buildings, and even tourists（在古蹟、建築、甚至遊客身上排泄）——鴿子多到把整個歷史城區占為己有，正是 take over。
前一句才說少了鴿子的歐洲城市之旅就不對味，本句以 In Barcelona, however 一轉，語氣從「城市風景」翻成「城市麻煩」；下一句 The city council originally proposed 減少鴿子數量的做法，也印證了問題的嚴重性。
set up 是「設立、架設」；hang on 是「稍等一下、堅持住」；break down 是「故障、破裂」。三者都無法表達「占據」。
take over（接管、占據）、take over from sb（接手某人的工作）、a takeover（收購）、take charge of（負責）、set up（設立）、hang on（稍等）。
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.