On the basis of the description given, classify each of the following solids as molecular, metallic, ionic, or covalent network. Explain your answers.
a. A lustrous, yellow solid that conducts electricity
b. A hard, black solid melting at 2350°C to give a nonconducting liquid
c. A nonconducting, pink solid melting at 650°C to give an electrically conducting liquid
d. Red crystals having a characteristic odor and melting at 171°C