Metallic solids are always opaque because

Carbon, silicon, and germanium have four valence electrons each. At room temperature, which one of the following statements is most appropriate?

If $V_0$ is the volume of a standard unit cell of germanium crystal containing $N_0$ atoms, then the expression for the mass $m$ of a volume $V$ in terms of $V_0, N_0, M$ and $N_A$ is [here, $M$ is the molar mass of germanium and $N_A$ is the Avogadro's constant].

$A$ pure semiconductor behaves slightly as a conductor at

$A$ piece of copper and another of germanium are cooled from room temperature to $80\, K$. Which of the following is a correct statement?

The forbidden energy band gaps in conductors,semiconductors,and insulators are $E_{G1}$,$E_{G2}$,and $E_{G3}$ respectively. The relation among them is: