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.
答案：(B) Therefore
正因為疫情擴散後控制困難，「因此」政府通常會積極採取緊急防疫措施，Therefore 表順接因果。Instead（取而代之）、By no means（絕不）、On the contrary（相反地）都不符合這裡的因果邏輯。
四個選項都是轉承語，位置一樣、形態一樣，你會憑中文挑一個。
這是邏輯勾環，線索一定要跨句才看得到。前一句是 When outbreaks become widespread within a country, control can be extremely difficult（疫情一旦在國內擴散，就極難控制）。空格這一句是 government authorities usually undertake aggressive emergency control measures as soon as an outbreak is detected（政府當局通常一偵測到疫情就立刻採取強力的緊急防控措施）。既然擴散之後很難控制，就得趁還沒擴散時趕快動手——前面是原因、後面是因應的做法，方向相同，所以用 Therefore（因此）。填進去讀成 Therefore, government authorities usually undertake aggressive emergency control measures。
Instead（反而、改為）是副詞，自己單獨站著、後面接逗號，形態上完全放得進去，容易被順手選走；但它要求前面那件事是被否定或被放棄的，而這一句並沒有推翻前一句，是順著它往下走。順帶把最常混的兩個一次分清楚：instead 是副詞，自己單獨用（I didn't go; I stayed home instead.）；instead of 是介系詞，後面一定要接名詞、代名詞或 V-ing（instead of going out）。柯林斯的 English Usage 特別提醒不可以寫成 instead to do something。By no means（絕不）是否定用語，放在句首時主要子句要倒裝，本句沒有倒裝；On the contrary（正好相反）要求後面推翻前面，方向不對。
therefore（因此）、instead（反而、改為，副詞自己單獨用）、instead of ＋ 名詞或 V-ing（而不是）、by no means ＋ 主要子句倒裝（絕不）、on the contrary（正好相反）、undertake measures（採取措施）。
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.