The Standard Error for the Difference in Means If Two $\sqrt { \frac { S _ { M} ^ { 2 } + S _ { W } ^ { 2 } } { n _ {M} + n _ { W } } }$

The standard error for the difference in means if two random variables M and W, when the two population variances are different, is

A) $\sqrt { \frac { S _ { M} ^ { 2 } + S _ { W } ^ { 2 } } { n _ {M} + n _ { W } } }$
B) $\frac { S _ { M} } { n _ {W } } + \frac { S _ { M } } { n _ { W } }$
C) $\sqrt { \sqrt { \frac { 1 } { 2 } \left( \frac { S _ { M } ^ { 2 } + S _ { W} ^ { 2 } } { n _ { M } + n _ {W} } \right) } }$
D) $\sqrt { \frac { S _ {M} ^ { 2 } + S _ { W} ^ { 2 } } { n _ { M } + n _ { W} } }$