Which of the Following Represents the Difference in Two Population $p _ { 1 } - p _ { 2 }$

Which of the following represents the difference in two population means?

A) $p _ { 1 } - p _ { 2 }$
B) $\mu _ { 1 } + \mu _ { 2 }$
C) $p _ { 1 } + p _ { 2 }$
D) $\mu _ { 1 } - \mu _ { 2 }$