5. A seven-digit telephone number $d_{1} d_{2} d_{3}-d_{4} d_{5} d_{6} d_{7}$, if the initial digits $d_{1} d_{2} d_{3}$ are in the same order as $d_{4} d_{5} d_{6}$ or $d_{5} d_{6} d_{7}$ (possibly all three are the same), it is called "memorable". If $d_{i}$ can be one of the digits $0,1,2,3, \cdots, 8,9$, then the number of different "memorable" numbers is
A. 19810
B. 19910
C. 19990
D. 20000
5. C Numbers in the form $d_{1} d_{2} d_{3}$ have $10^{3}=1000$ variations.
When $d_{1} d_{2} d_{3}$ is the same as $d_{4} d_{5} d_{6}$, $d_{7}$ can have 10 different forms; when $d_{1} d_{2} d_{3}$ is the same as $d_{5} d_{6} d_{7}$, $d_{4}$ can have 10 different forms; and when $d_{1} d_{2} d_{3}$ is the same as both $d_{4} d_{5} d_{6}$ and $d_{5} d_{6} d_{7}$, it is clear that
$$
d_{1}=d_{2}=d_{3}=d_{4}=d_{5}=d_{6}=d_{7},
$$

in which case there are also 10 different forms.
By the principle of inclusion-exclusion, the number of such numbers is
$$
1000 \times 10 + 1000 \times 10 - 10 = 19990.
$$

Therefore, the answer is $\mathrm{C}$.