Read the following passage and answer the question given below <br> <br>
Some of the conventional sources of energy have poor efficiency and produce a lot of smoke because of incomplete combustion. A few examples are wood, animal dung, agricultural wastes, etc; <br><br>
Wood has poor efficiency because only 8% of the wood is utilized as fuel. The results in incomplete combustion. Also, it produces gases like carbon di oxide which is hazardous to our health. <br><br>
Animal dung when mixed with mud and made in the form of cakes, which are then dried and burnt are also used for domestic purpose. As it burns, it produces a lot of smoke and hence results in air pollution. <br><br>
Agricultural wastes lie sugarcane are burnt for use in industries. This is more preferable for biogas plants.
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QUESTION : The smoke which is produced by the conventional source is due to
