Which of the following is a true alum?

The structure of diborane $(B_2H_6)$ contains :-

Which of the following on hydrolysis produces diborane $(B_2H_6)$?

$TlI_3$ is isomorphous with:

The correct statements from the following are:
$(A)$ The decreasing order of atomic radii of group $13$ elements is $Tl > In > Ga > Al > B$.
$(B)$ Down the group $13$ electronegativity decreases from top to bottom.
$(C)$ $Al$ dissolves in dil. $HCl$ and liberates $H_2$,but conc. $HNO_3$ renders $Al$ passive by forming a protective oxide layer on the surface.
$(D)$ All elements of group $13$ exhibit a highly stable $+1$ oxidation state.
$(E)$ Hybridisation of $Al$ in $[Al(H_2O)_6]^{3+}$ ion is $sp^3d^2$.
Choose the correct answer from the options given below:

Explain the following:
$(1)$ $BF_3$ does not hydrolyze.
$(2)$ Why does the element silicon not form a graphite-like structure,whereas carbon does?