Haloalkanes and Haloarenes

For the given pair of $${ S }_{ N }2$$ reactions, indicate which occurs with a larger rate constant.
Calculate total number of optical isomers in following compound is $$CH_3(CHOH)_3CH_3$$
Compound (A) can not be directly brominated due to
Chlorination of an alkane as compared to bromination proceeds? 
Breathing about 900 parts this substance per million parts of air for a short time cause dizziness heart & fatigue headache is-