Dear student.
Moisture is required for growth of microorganisms and their reproduction. In absence of moisture, these organisms cannot replicate. Hence dry food items will not get spoiled in airtight containers. Secondly, refrigerator prevents food spoilage as microbes do not get the optimum temperature for their growth.
This shows that Mrs Sharma is quite aware of how to keep food safe and thus preventing her family member from food poisoning. Mrs Sharma is a knowledgeable person.
Regards.