Which of the following substances is used in smoke screens?

Evaluate the following statements related to group $14$ elements for their correctness.
$(A)$ Covalent radius decreases down the group from $C$ to $Pb$ in a regular manner.
$(B)$ Electronegativity decreases from $C$ to $Pb$ down the group gradually.
$(C)$ Maximum covalence of $C$ is $4$ whereas other elements can expand their covalence due to presence of $d$ orbitals.
$(D)$ Heavier elements do not form $p\pi-p\pi$ bonds.
$(E)$ Carbon can exhibit negative oxidation states.
Choose the correct answer from the options given below:

Which one of the following is a semiconductor?

Consider the following reaction:
$R_2SiCl_2 + H_2O \to (1) \xrightarrow{\text{polymerisation}} (2)$
Compound $(2)$ in the above reaction is:

Among the following oxides of $p$-block elements,the number of oxides having amphoteric nature is: $Cl_2O_7, CO, PbO_2, N_2O, NO, Al_2O_3, SiO_2, N_2O_5, SnO_2$

$A$ major constituent of Portland cement,except lime,is: