What is the equation used to obtain I(Intensity) component of each RGB pixel in RGB color format?

What is the equation used to obtain I(Intensity) component of each RGB

1.	What is the equation used to obtain I(Intensity) component of each RGB pixel in RGB color format?
A.	I=1/2(R+G+B)
B.	I=1/3(R+G+B)
C.	I=1/3(R-G-B)
D.	I=1/3(R-G+B)
Answer» C. I=1/3(R-G-B)