no idea abt statA.
statB is obviously true.
A:The rate of loss of optical activity during isotopic inversion SN2 is twice the rate of isotope exchange.
B:SN2 reaction proceeds through inversion
(i)Both true,B is the correct explanation of A
(ii)Both true B is not the correct explanation of A
(iii)A is true,B is false
(iv)B is true,A is false
100% loss in optical activity means 100% racemisation and no extra inversion. percentage of backside attack = percent of inversion + 1/2*(%age of racemisation)=0 + 1/2*100=50
so 50% backside attack would give 100% racemised product.Each backside attack so generates a pair of racemised molecule or the loss of optical activity is twice the rate of isotopic exchange
source: MTG reacn mechanisms by Udala C Ray