Economic consequences are often most serious in developing countries 29 poultry raising is an important source of income, and of food, for impoverished rural farmers and their families.
答案：(D) where
where 引導形容詞子句修飾地方先行詞 developing countries，表示「在那些國家（養禽業是重要收入來源）」。that 通常不直接引導完整地方子句、until（直到）、although（雖然）都不符合這裡「修飾地點」的語法功能。
四個選項裡有 that 也有 where，中文都能說成「那個」「那裡」，你會憑語感挑一個。
答對率四成三。關係詞的題目一律用同一招：把先行詞代回子句裡，看它在子句裡變成什麼形式。這是關係代名詞的定義本身——它就是代替前面出現過的那個名詞，所以把名詞放回去，答案自己就出來了。先行詞是 developing countries（開發中國家），子句是 poultry raising is an important source of income（養禽是重要的收入來源）。把「開發中國家」代回去，句子要寫成 poultry raising is an important source of income in those countries——國家在子句裡是「在那些國家」這個地方副詞，前面得補一個介系詞 in，既不是主詞也不是受詞。代回去變成地方副詞的，就用關係副詞 where。填進去讀成 in developing countries where poultry raising is an important source of income。代回去長什麼樣就用哪一個：當主詞用 who 或 which、當受詞用 whom 或 which、變成所有格用 whose、變成地方副詞用 where、變成時間副詞用 when。
that 是關係代名詞，代回去要能直接當主詞或受詞，但這一句代回去是 in those countries，少不了那個 in，所以 that 接不上（真要用也得寫成 in which）；until（直到）與 although（雖然）是連接詞，填進去語意都不通。
where（關係副詞，先行詞代回子句後變成地方副詞）、in which 等於 where（在其中）、whose（關係代名詞的所有格，代回子句後變成所有格）、a source of income（收入來源）、impoverished（貧困的）、rural（鄉村的）。
Avian influenza, or "bird flu," is a contagious disease caused by viruses that normally infect only birds and, less commonly, pigs. 26 all bird species are thought to be susceptible to infection, domestic poultry flocks are especially vulnerable to infections that can rapidly reach epidemic proportions.
Outbreaks of avian influenza 27 be devastating for the poultry industry and for farmers. For example, an outbreak of avian influenza in the USA in 1983–1984 resulted in the destruction of more than 17 million birds 28 nearly US$ 65 million. Economic consequences are often most serious in developing countries 29 poultry raising is an important source of income, and of food, for impoverished rural farmers and their families.
When outbreaks become widespread within a country, control can be extremely difficult. 30, government authorities usually undertake aggressive emergency control measures as soon as an outbreak is detected.