Correct statements regarding $A{l}_{2}C{l}_6$ is/are ___?

a)The covalency of Al in $A{l}_{2}C{l}_6$ is 4

b) $AlC{l}_3$ molecules are dimerized through halogen bridging

c)In $A{l}_{2}C{l}_6$ the number of covalent bonds and co-ordinate covalent bonds are 6 and 2