7. The program flowchart is shown in Figure 3. Given that the real number $x \in [1, 9]$. Then the probability that the output $x$ is not less than 55 is ( ).
(A) $\frac{1}{3}$
(B) $\frac{2}{3}$
(C) $\frac{3}{8}$
(D) $\frac{5}{8}$
7. C.

It is easy to know that when $x=6$, the output result is $x=55$. Therefore, the required probability is the case when $x \geqslant 6$. Note that, this problem fits a geometric distribution. Thus, $p=\frac{3}{8}$.