To evaluate the planting effectiveness of a crop, $n$ plots of experimental land were selected. The per-acre yields (in kg) of these $n$ plots are $x_1, x_2, \cdots, x_n$ respectively. Among the following indicators, which can be used to evaluate the stability of this crop's per-acre yield?
A. Mean of $x_1, x_2, \cdots, x_n$
B. Median of $x_1, x_2, \cdots, x_n$
C. Maximum value of $x_1, x_2, \cdots, x_n$
D. Standard deviation of $x_1, x_2, \cdots, x_n$