Properties about mercury the element

1. Mercury is the only metallic element that is liquid at standard conditions for temperature and pressure. 2. It is a poor conductor of heat, but a fair conductor of electricity. 3. Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate, nitrate, and chloride. 4. Mercury dissolves many other metals such as gold and silver to form amalgams. 5. It has a freezing point of −38.83 °C and a boiling point of 356.73 °C. Hope these 5 properties are enough :)

C) the increase in atmospheric pressure pushes the walls of the balloon outward thus making it  is the answer.hope i you.