120 The number of real roots of the equation $2 \cos \frac{x}{2}=10^{x}+10^{-x}+1$ is
A. 0
B. 1
C. 2
D. 3
120 A. Since $10^{x}+10^{-x}+1 \geqslant 3 \cdot 3 \sqrt{10^{x} \cdot 10^{-x} \cdot 1}=3$ (equality holds only when $x=0$), and $2 \cos \frac{x}{2} \leqslant 2$, there does not exist a real number $x$ such that $2 \cos \frac{x}{2}=10^{x}+10^{-x}+1$.