98. Most commercial pesticides are effective for only 2-3 years. This is because?

The closest correct answer would be "c". Actual resistance to pesticides is not all that common, especially with the newer generations of compounds that are used in agriculture and professional applicators. All pesticides degrade over time, they may be affected by UV light, water and temperature but over time the become biologically inactive and frankly are designed to do so as not to become persistant in the environment as did DDT and other cyclodienes.