Correct statements regarding ${\mathrm{Al}}_2{\mathrm{Cl}}_6$

a) The covalency of Al in ${\mathrm{Al}}_2{\mathrm{Cl}}_6$ is 4

b) ${\mathrm{AlCl}}_3$ molecules are dimerised through halogen bridging

c) In ${\mathrm{Al}}_2{\mathrm{Cl}}_6$ the number of covalent bonds and co-ordinate covalent bonds are 6 and 2