7. There are ( ) integers $n$ such that $400\left(\frac{2}{5}\right)^{n}$ is an integer.
(A) 3
(B) 4
(C) 6
(D) 8
(E) 9
7. E.

Notice,
$$
400\left(\frac{2}{5}\right)^{n}=2^{5} \times 5^{3}\left(\frac{2}{5}\right)^{n}=2^{n+5} \times 5^{3-n} \text {, }
$$

where, $n+5$ and $3-n$ are both non-negative integers.
Therefore, $n=-5,-4, \cdots, 3$, a total of 9.