Which statements are true for `S_(N)2` reaction of alkyl halides?
i) Both of the alkyl halide and nucleophile are involved in the transition state
ii) Reaction proceeds with inversion of configuration at the substitution centre.
iii) Reaction proceeds with retention of configuration at the substitution centre.
iv) the order of reactivity is `3^(0)>2^(0)>1^(0)` .
v) the nucleophile must have an unshared electron pair and bear a negative charge.
vi) the greater the nucleophilicity of the nucleophile, the greater the rate of reaction.