Decide, among the following sets, which sets are subsets of one and another: A = \{ x : x \in \mathbb{R} \text{ and } x \text{ satisfy } x^2 - 8x + 12 = 0 \}, B = \{ 2, 4, 6 \}, C = \{ 2, 4, 6, 8, \ldots \}, D = \{ 6 \}