The elements of the second period in the modern periodic table are given below. Refer the table and answer the questions given below :

Group	1	2	13	14	15	16	17	18
Period	Li	Be	B	C	N	O	F	Ne

What is the valency of Boron?

Given below are two statements: one is labelled as Assertion $\mathrm{A}$ and the other is labelled as Reason $\mathrm{R}$.

Assertion $\mathrm{A}:$ In $\mathrm{T}l{\mathrm{I}}_3,$ isomorphous to ${\mathrm{CsI}}_3$, the metal is present in $+1$ oxidation state.

Reason $\mathrm{R}:$ $\mathrm{T}l$ metal has fourteen $\mathrm{f}$ electrons in its electronic configuration.

In the light of the above statements, choose the most appropriate answer from the options given below:

Identify the correct statements about the anomalous behaviour of Boron

I. Boron trihalides can form dimeric structures.

II. Boron shows $+1$ as stable oxidation state

III. Maximum covalency of Boron is four.

IV. Boron does not form ${\mathrm{BF}}_6^{6-}$ ion.

Given below are two statements: one is labelled as Assertion $\mathrm{A}$ and the other is labelled as Reason $\mathrm{R}$

Assertion $\mathrm{A}$: Boron is unable to form ${\mathrm{BF}}_6^{3-}$

Reason $\mathrm{R}$: Size of $\mathrm{B}$ is very small.

In the light of the above statements, choose the correct answer from the options given below