The Test Scores of 40 Students Are Summarized in the Frequency

Multiple Choice

The test scores of 40 students are summarized in the frequency distribution below. Find the standard deviation.
$\begin{array}{r|c}\text { Score } & \text { Students } \\\hline 50 \text {-under } 60 & 8 \\60 \text {-under } 70 & 15 \\70 \text {-under } 80 & 6 \\80 \text {-under } 90 & 5 \\90 \text {-under } 100 & 6\end{array}$

A) $13.5$
B) $12.2$
C) $14.2$
D) $12.8$