In the reaction :
`R-Br+CI^(-) to R -CI+Br^(-)`
The rates of reaction of ethyl bromide (I) n-propyl bromide (II) isobutyl bromide (III) and neopentyl bromide (IV) follow the order:
A. `IV gt III gt II gt I`
B. `I gt II gt III gt IV`
C. `I gt III gt II gt IV`
D. `III gt II gt IV gt I`