第 16 至 20 題為題組
        Street pigeons are a common sight in many European cities. In fact, a visit to a major European cityjust 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’shistoric center, pooping on monuments, buildings, and even tourists.
        The city council originally proposed   18    the pigeon population by gathering the birds andshooting them. But that idea led to strong protests from numerous animal rights groups. After heated debatesand 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 toblock 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 pigeonpopulation in the first year and between 70 and 80 percent in four or five years.
【題組】16.
(A) as
(B) for
(C) though
(D) without