(A) : Boron can be used as refractory solid.
(R) : Boron has very high melting point.

The materials which are resistant to high temperatures are said to be Refractory materials, which are used predominantly as furnace linings for elevated temperature process.

Boron has high melting point and hence it can be used as Refractory material.

Boron has very high melting point because of its small atomic size, it forms strong covalent bonds with the neighboring atoms. Thus, boron atoms are closely packed in its solid state, so a large amount of heat is needed to break the bonds between atoms.