Why is carbon monoxide production undesirable in furnace operation?

Why is carbon monoxide production undesirable in furnace operation?
 
  A) Carbon monoxide can cause illness. B) Carbon monoxide is an inert substance.
  C) Carbon monoxide causes furnace inefficiency. D) Carbon monoxide may block ventilation.

 Q. 2

What is the lowest acceptable overvoltage category rating for a voltage tester that will be used to test electronic
  equipment and circuitry?
 
  a. CAT I
  b. CAT II
  c. CAT III
  d. CAT IV

 Q. 3

A small lapse in welder technique and concentration could lead to weld ________, such as lack of fusion.
 
  A) A bird's nest
  B) Cracks
  C) Defects
  D) CO2 leaks
  E) None of the above