1. 'CO' stands for 'corona', 'VI' for 'virus', and 'D' for 'disease'. The word 'for' is used to indicate the meaning of an acronym or abbreviation, so the correct answer is A. 'for'.

2. The sentence is comparing COVID-19 symptoms to those of other conditions, so it requires the adjective form of 'similar'. The correct answer is D. 'similar'.

3. The sentence is referring to people who have been infected with COVID-19 but don't have any symptoms. The correct relative pronoun to use in this context is 'who'. The correct answer is A. 'who'.

4. The sentence is listing some of the symptoms of COVID-19, and 'loss of taste or smell' is one of them. The correct answer is C. 'loss'.

COVID-19 is an infectious respiratory illness caused by a coronavirus called SARS-CoV-2. ‘CO’ stands for corona, ‘VI’ for virus, and ‘D’ for disease.

Many COVID-19 symptoms are similar to those of the flu, the common cold and other conditions, so a test is required to confirm if someone has COVID-19. Symptoms may appear 2 to 14 days after exposure to the virus and can range from very mild to severe illness. Some people who have been infected don’t have any symptoms.

The most common symptoms are fever, cough, and tiredness. Other symptoms can include shortness of breath, chest pain or pressure, muscle or body aches, headache, loss of taste or smell, confusion, sore throat, congestion or runny nose, diarrhea, nausea and vomiting, abdominal pain, and skin rashes. In addition to these symptoms, infants may have difficulty feeding.

Children of any age can become ill with COVID-19. While children and adults experience similar symptoms, children generally have less serious illness than adults.

25. B 
Giải thích: stand for (n): viết tắt. Trong câu đó dịch là: 'CO' là viết tắt của corona
26. B
Giải thích: Dùng mẹo nhé. Cậu thấy "many" với "are" đúng không? Chắc chắn đằng sau "are" là danh từ số nhiều 
27. A
Giải thích: Some people (S) + who + have been infected (V) + ...
Ngoài ra, ở đây mình cũng dùng biện pháp loại trừ. Which thì không hợp hoàn cảnh, whome thì không đúng cấu trúc, where thì lại càng không.
28. C
Giải thích: loss (n): mất đi. Loss of taste or smell... nghĩa là mất vị giác hay khứu giác