A history of trauma to the head with a variable period of unconsciousness or confusion and then a return to normal consciousness with possible amnesia best explains a(n):

Patients who are in a coma:

a. Are unaware of their surroundings b. Can respond to stimuli c. Usually have their eyes open d. Can respond to simple commands

Which of the following substances is found in high amounts in calculus?

A) Fluoride B) Silica C) Hydroxyapatite D) Tricalcium phosphate