It is widely accepted that China is a country faced with severe water shortages. Insuffici

ent water resources have slowed agricultural development. And to make matters worse, some of the traditional Chinese irrigation methods have wasted an astonishing amount of water.

In China today, the utilization efficiency of farming water is about 30--40 per cent. This figure stands in sharp contrast to developed countries' utilization average of 70--80 per cent. The low utilization efficiency has resulted from the adoption of some traditional Chinese irrigation methods.

Only by using modern irrigation methods can we reduce water shortage in agriculture.

One of the advantages of modern irrigation methods is that they alone can save 20--30 per cent of the present volume of wasted irrigation water.

The main topic of the passage is______.

A．agricultural products.

B．irrigation methods.

C．natural resources.

D．water shortages.