The passage would most likely be found in a texbook on which of the following subjects

A．countries of the world will work together on pollution problems
B．some industries are now making economic use of liquid wastes
C．by-products from wastes lead to a more prosperous marketplace
D．science is making great progress in increasing water supplies

A．to acquaint the reader with water pollution problems
B．to alert the reader to the dwindling water supply
C．to explain industrial uses of water
D．to demonstrate various measures to solve the pollution problem