Which of the following statements is true about $H_3BO_3$?

The crystalline form of borax has
$(A)$ tetranuclear $\left[B_4O_5(OH)_4\right]^{2-}$ unit
$(B)$ all boron atoms in the same plane
$(C)$ equal number of $sp^2$ and $sp^3$ hybridized boron atoms
$(D)$ one terminal hydroxide per boron atom

Group $13$ elements show $+1$ and $+3$ oxidation states. The relative stability of the $+3$ oxidation state is given as:

Marine plants are an important source of which of the following?

Boric acid is an acid because its molecule

Three pairs of compounds are given below. Identify the compound in each pair which has the group $13$ element in a more stable oxidation state. Give reasons for your choice and state the nature of bonding. $(A)$ $TlCl_3, TlCl$ $(B)$ $AlCl_3, AlCl$ $(C)$ $InCl_3, InCl$