Arrange the following halides in the decreasing order of `S_(N^(1))` reactivity : `underset((I))(CH_(3)CH_(2)CH_(2)Cl), underset((II)) (CH_(2)=CHCH(Cl)CH_(3)), underset((III))(CH_(3)CH_(2)CH(Cl)CH_(3))`
A. I gt II gt III
B. II gt I gt III
C. II gt III gt I
D. III gt II gt I