10. As shown in Figure 1, a carpet has three different colors, and the areas of the three differently colored regions form an arithmetic sequence. The width of the smallest rectangle in the middle is 1 foot, and the surrounding width of the other two shaded areas is also 1 foot. Then the length of the smallest rectangle in the middle is ( ) feet.
(A) 1
(B) 2
(C) 4
(D) 6
(E) 8
10. B.

Let the length of the central small rectangle be $x$ feet.
Then the area of the small rectangle is $x \cdot 1$, the area of the middle region is $(x+2) \times 3 - x = 2x + 6$, and the area of the outermost region is $(x+4) \times 5 - (x+2) \times 3 = 2x + 14$. Since the areas of the three parts form an arithmetic sequence, we have $x + 2x + 14 = 2(2x + 6) \Rightarrow x = 2$.