Which of the following statement is not correct for nucleophilic substitution reaction ?
A. correct decreasing order for `S_(N)1` reactivity is `(CH_(3))_(3)C-Clgt(CH_(3))_(2)CHClgtCH_(3)CH_(2)Cl`
B. Stereochemical inversion occurs in `S_(N)2` reaction
C. Racemisation occurs in `S_(N)1` reaction
D. Carbocation is formed during `S_(N)2` reaction