A Statistics Teacher Wants to See If There Is Any $\begin{array}{ll}&\text { Today } & \text { Five Years Ago } \\\bar {x}&82 & 88 \\\sigma^2&112.5 & 54 \\n&45 & 36\end{array}$

Multiple Choice

A statistics teacher wants to see if there is any difference in the abilities of students enrolled in statistics today and those enrolled five years ago.A sample of final examination scores from students enrolled today and from students enrolled five years ago was taken.You are given the following information. $\begin{array}{ll}&\text { Today } & \text { Five Years Ago } \\\bar {x}&82 & 88 \\\sigma^2&112.5 & 54 \\n&45 & 36\end{array}$
The standard error of $\bar { x } _ { 1 }$ - $\bar { x } _ { 2 }$ is

A) 12.9.
B) 9.3.
C) 4.
D) 2.