\(\begin{array}{c|c}
\text{Units of Quantity consumed} & \text{Total utility} & \text{Marginal utility} \\
0 & --- & --- \\
1 & 10 & 10 \\
2 & 15 & 5 \\
3 & 17 & 2 \\
4 & 18 & 1 \\
5 & 18 & 0 \\
\end{array}\)
The table above illustrates the law of?