$\text{1.}$ C 

$\rightarrow$ Dẫn chứng :  Dựa vào đoạn văn thứ nhất của bài văn 

$\text{2.}$ B

$\rightarrow$Dẫn chứng :  A few learn English just by hearing the language in films, on telivision, in the office or among their friends.

$\text{3.}$ C 

$\rightarrow$Dẫn chứng : Many boys and girls learn English at school because it is one of their subjects. 

$\text{4.}$ B 

$\rightarrow$Dẫn chứng : Many adults learn English because it is useful for their work. 

$\text{5.}$ D

$\rightarrow$Dẫn chứng :  Dựa vào cả bài văn .

=>
1. D (Every year people in many countries learn English)
2. C (Most people must work hard to learn English)
3. A (Many boys and girls learn English at school because it is one of their subjects)
4. B (Many adults learn English because it is useful for their work)
5. D English is difficult to learn