Microorganisms : Friend and Foe

Give reasons for the following.
(a) Fresh milk is boiled before consumption while processed milk
stored in packets can be consumed without boiling.
(b) Raw vegetables and fruits are kept in refrigerators whereas
jams and pickles can be kept outside.
(c) Farmers prefer to grow beans and peas in nitrogen deficient
soils.
(d) Mosquitoes can be controlled by preventing stagnation of
water though they do not live in water. Why?