The number of $2$-centre-$2$-electron and $3$-centre-$2$-electron bonds in $B_2H_6$,respectively,are

Which one of the following is the correct statement?

The number of $P-H$ bond$(s)$ in $H_3PO_2, H_3PO_3$ and $H_3PO_4$,respectively,is

Given below are two statements:
Statement $I$: Boron is extremely hard,indicating its high lattice energy.
Statement $II$: Boron has the highest melting and boiling point compared to its other group members.
In the light of the above statements,choose the most appropriate answer from the options given below:

Give the physical and chemical properties of orthoboric acid $(H_3BO_3)$.

Why is boric acid considered as a weak acid?