Pickles have a long shelf life and do not get spoiled for months, why?

Packet milk does not get spoiled for longer time, why?

Though the number of physical quantities is very large yet we do not have as many different units, why?

Which cells of tissues do not have division power and why?

Why do high altitude areas have brighter sunlight and lower temperatures as compared to the plains.

A farmer harvests his crop and expresses his harvest in three different ways: (a) I have harvested 10 quintals of wheat. (b)1 have harvested 10 quintals of wheat today in one acre of land. (c) I have harvested 10 quintals of wheat in one acre of land after 6 months from sowing. Do the above statements mean one and the same thing. If your answer is yes, give reasons. And if your answer is 'no' explain the meaning of each expression.