A : Be is sp hybridized in `BeCl_(2)` at room temperature
R : Be is `sp^(3)` hybridized above 1200 K in `BeCl_(2)`

Explanation

(a). The vapour state, `BeCl_(2)` exists as a linear molecule, i.e. each `Be` is bonded to two `Cl` atoms and `Be` is `sp` hybridised.
Whereas in the solid state, `BeCl_(2)` exists as a polymer and each `Be` is bonded to two `Cl` atoms by covalent bond and two `Cl` atoms by coordinate bond. `Be` atom in solid `BeCl_(2)` is `sP^(3)` hybridised.
(b). In vapour state, `BeCl_(2)` has chlorobridged dimer structure which dissociates into linear monomer at `1000^(@)C`

(c ). The monomer `BeH_(2)` and `BeCl_(2)` formed with normal covalent bonds will result in only four electrons in the valence shell of beryllium and thus they are electron-deficient compounds. By polymerising, each atom shares its electons with several neighbours and recieves a share in their electorn making the situation more favourable. Therefore, the hydrides and halides of `Be` polymerise.