Described as invisible in the opposite of gravity

Normal Force

Explanation:

Invisible Forces covers the topics of magnetism and static electricity, two invisible yet powerful forces that children experience, often unwittingly, every day. Its focus is non-contact forces, that is, forces that can affect objects without touching them. It also looks at current electricity as a source of magnetism. The opposite of gravity is called normal force, which is denoted by “R” or “N”. According to Newton's 3rd law every action has an equal & opposite reaction.

i'm sure the answer you are looking for would be

a. an ice age

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