Which of the following chlorides does not react with $PCl_5$ on heating?

$A$ and $B$ are formed when borax is dissolved in water. $C$ and $B$ are formed when borax is reacted with aqueous $HCl$ solution. What are $A$ and $C$ respectively?

What is the product formed by the action of caustic soda on aluminum hydroxide?

When borax is heated with $CoO$ on a platinum loop,the blue coloured bead formed is largely due to

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

Given below are two statements :
Statement $I$ : Gallium is used in the manufacturing of thermometers.
Statement $II$ : $A$ thermometer containing gallium is useful for measuring the freezing point $(256 \ K)$ of brine solution.
In the light of the above statement,choose the correct answer from the options given below :