Prehistory can be described as
a. what happened last year
b. what happened millions of years ago
c. the time prior to the use of written records

Prehistory is the time before history.

History is usually defined as the time in the past BEFORE written (or similar) solid evidence, for which we can only infer what happened.

So the correct answer is C.

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later