Each of the following questions has a paragraph from which one sentence has been deleted. From the given options, choose the one that completes the paragraph in the most appropriate way. Public health systems play a critical role in preventing disease and protecting community well-being. Vaccination programmes, sanitation infrastructure, and health education campaigns have historically reduced the spread of infectious illnesses. These measures are particularly important in densely populated urban areas where diseases can spread rapidly. (______________). When prevention strategies fail, health systems may become overwhelmed by sudden outbreaks.