What is the formula of Carborundum?

Which of the following substances is used in smoke screens?

In the laboratory,silicon can be prepared by the reaction of:

$(a)$ Classify the following oxides as neutral,acidic,basic,or amphoteric:
$CO, B_2O_3, SiO_2, CO_2, Al_2O_3, PbO_2, Tl_2O_3$
$(b)$ Write suitable chemical equations to show their nature.

Explain the following:
$(a)$ $CO_2$ is a gas whereas $SiO_2$ is a solid.
$(b)$ Silicon forms $SiF_6^{2-}$ ion whereas corresponding fluoro compound of carbon is not known.

Consider the following route of reactions $R_2SiCl_2 + H_2O \to (A) \xrightarrow{\text{Polymerisation}} (B)$. Compound $(B)$ in the above reaction is: