Questions 19 to 21 are based on the conversation you have just heard.

A．Because farmers use less insecticides now.
B．Because farmers have been using insecticides for many years.
C．Because insects use the mechanisms against plants to fight insecticides.
D．Because insects have been eating plants for many million years.

A．There is a greater demand of clean water.
B．More water sources are polluted or disappearing.
C．Developed countries consume too much water.
D．More water is ruined and developed countries consume too much.