A challenge is something that requires great physical or mental to overcome. When answering a 'challenge' question, you must put emphasis on why the problem is so hard to overcome that it becomes a challenge. The easiest way to do it is by saying that an existing measure has been done, but it is still inadequate. Please look at the example below for more explanation.
What is the biggest difference between these two answers?
Answer B focuses on the characteristics of the setting in Hong Kong, e.g. high population density. Then It mentions some existing measures. Lastly, it explains why they are still not enough, which then brings out the real challenges.
Answer A explains the problems of controlling Covid-19, but they are problems rather than challenges. Also, there is no indication that the answer addresses the term 'zero-infection', i.e. not simply reducing the case number.