The use of water closet and septic tanks is preferred to the use of bucket and pit latrines because

Analyze the question: It asks why water closets (WC) and septic tanks are better than bucket and pit latrines. The answer should relate to public health and the spread of disease.

Concept: Faeces can carry the germs of cholera, typhoid, dysentery, and worm eggs. Flies feed and breed on exposed faeces and then carry the germs to food and water. A good sanitation system must therefore keep faeces sealed off from flies and other disease carriers.

Examining each option:

• A. Cost effective: This is wrong. Water closets and septic tanks are expensive to build and maintain. Pit latrines and buckets are cheaper.
• B. Contaminated faeces are completely kept away from flies: This is correct. In a water closet, faeces are flushed away immediately. They go into a closed septic tank, so flies cannot reach them. Bucket and pit latrines often leave faeces exposed, so flies can land on them and spread disease.
• C. Flushing water contains chlorine: This is wrong. Flushing water is not necessarily chlorinated, and chlorine is not the main reason for the preference.
• D. It prevents flies from breeding: This is too strong. Flies can still breed in poorly maintained septic systems, and the main benefit is that faeces are kept out of reach of flies, not that breeding is prevented.

Tip: In sanitation questions, look for the option about isolating faeces from flies and water sources. Be careful with words like "prevents" or "completely" in an option, but here "kept away from flies" is the best fit for the main idea.