7. If $a^{2}-14 a+1=0$, then the tens digit of $a^{4}+\frac{1}{a^{4}}$ is ( ).
(A) 2
(B) 3
(C) 4
(D) 5
7.B.
$$
a+\frac{1}{a}=14, a^{2}+\frac{1}{a^{2}}=14^{2}-2=194 \text{, }
$$
- $a^{4}+\frac{1}{a^{4}}=194^{2}-2$.

Then $194^{2}-2=(200-6)^{2}-2 \equiv 34(\bmod 100)$.