In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.

When the temperature (1)______ during the summer season, many people choose to use airconditioners to keep their homes and offices cool. Air-conditioners, which were once regarded as a luxury, have now almost become a/an (2)______. The air-conditioning system not only cools the indoor air down but also filters out harmful pollutants. This helps reduce dust, pollen and allergens inside the home and (3)______ the quality of air inside the house or room. However, along with the (4)______, there are some disadvantages. A major disadvantage is that if the filter inside the air-conditioner is not cleaned and maintained regularly, it (5)______ to problems such as allergic colds, fever, headaches and dryness of the skin.