What can a drop of liquid mercury be described as?
a) a pure substance and a compound
b) an impure substance that is reactive
c) an impure substance that is unreactive
d) a pure substance and an element

Adrop of mercury can be defined as a pure substance and and element, so your answer is d.

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