why not 18?

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Given an instance of the STUDENTS relation as shown as below

$$\begin{array}{|c|c|c|c|c|} \hline \textbf {StudentID} & \textbf{StudentName} & \textbf{StudentEmail} & \textbf{StudentAge} & \textbf{CPI} \\\hline \text{2345} & \text{Shankar} & \text{[email protected]} & \text{X} & \text{9.4}\\\hline\text{1287} & \text{Swati} & \text{[email protected]} & \text{19} & \text{9.5}\\\hline\text{7853} & \text{Shankar} & \text{[email protected]} & \text{19} & \text{9.4} \\\hline \text{9876} & \text{Swati} & \text{[email protected]} & \text{18} & \text{9.3} \\\hline \text{8765} & \text{Ganesh} & \text{[email protected]} & \text{19} & \text{8.7} \\\hline \end{array}$$

For $(\text{StudentName, StudentAge})$ to be a key for this instance, the value $X$ should NOT be equal to______.

$$\begin{array}{|c|c|c|c|c|} \hline \textbf {StudentID} & \textbf{StudentName} & \textbf{StudentEmail} & \textbf{StudentAge} & \textbf{CPI} \\\hline \text{2345} & \text{Shankar} & \text{[email protected]} & \text{X} & \text{9.4}\\\hline\text{1287} & \text{Swati} & \text{[email protected]} & \text{19} & \text{9.5}\\\hline\text{7853} & \text{Shankar} & \text{[email protected]} & \text{19} & \text{9.4} \\\hline \text{9876} & \text{Swati} & \text{[email protected]} & \text{18} & \text{9.3} \\\hline \text{8765} & \text{Ganesh} & \text{[email protected]} & \text{19} & \text{8.7} \\\hline \end{array}$$

For $(\text{StudentName, StudentAge})$ to be a key for this instance, the value $X$ should NOT be equal to______.

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