The city council originally proposed 18 the pigeon population by gathering the birds and shooting them.
答案：(B) reducing
語意「市議會原本提議透過聚集並射殺鴿子來『減少』鴿子數量」，reducing（減少）符合，跟後文槍殺鴿子的手段呼應；counting（計算）、displaying（展示）、maintaining（維持，語意相反）都不合。
四個選項都是動名詞，接在 proposed 後面文法上都成立，所以只能比語意。你可能被 counting（計算）吸引，因為後面提到 gathering the birds（把鳥聚集起來），聽起來像要清點數量。
關鍵在 gathering 後面接的那個動詞：and shooting them（然後射殺牠們）。聚集起來是為了射殺，射殺的目的當然是減少數量，不是清點。 再往後看最後一句 there would be a decrease of 20 percent in the pigeon population（鴿子數量會減少兩成），decrease 這個字直接印證整段的目標就是 reduce。
counting（計算）與射殺這個手段搭不起來；displaying（展示）語意不通；maintaining（維持）方向相反，市議會要的是減少不是維持現狀。
reduce the population（減少數量）、a decrease of 20 percent in（……減少兩成）、propose Ving（提議做……）、birth control（節育）、maintain（維持）、animal rights group（動物權益團體）。
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.