18. If illumination varies inversely to the square of its distance from the light source, which would likely happen if the distance from a light source were increased to twice its original value?

A. The illumination will be doubled.
B. The illumination will be halved.
C. The illumination will be reduced to one-fourth of its original value.
D. The illumination will be increased four times of its original value

20. Which of the following refers to an object that emits its own light?

A. luminous object
B. illuminated object
C. transparent
D. incandescence