Important roles of bacteria include
decomposition
significance in commercial products such as cheese and yogurt
sewage disposal
nitrogen fixation

All the four

Bacteria are single celled simple prokaryotic organisms. They perform different roles in nature:

1. decomposition: It is a process in which the complex organic matter from dead remains of plants and animals is converted into simple molecules. Many bacterial agents participate in this process, in order to obtain their nourishment. Decomposition is a natural waste management process.

2.significance in commercial products such as cheese and yogurt: Bacterial strain named as Lactobacillus is used in dairy products like cheese and yogurt. Bacteria are used to make organic acids in pickles and vinegar. Sourdough bread is made by the fermentation process of the yeast and bacteria is also used as an ingredient.

3.sewage disposal: The decomposing nature of the bacteria facilitates the degradation of organic waste present in the sewage. Therefore, they help in sewage waste management or disposal.

4. nitrogen fixation: Some bacteria lives in symbiotic relationship with plants, especially with leguminous plants. The bacteria trap the atmospheric nitrogen from the environment and supply it to plants, which is necessary for growth, in return the bacteria receives nutrition from plants.

Therefore, all the four options are correct.

the answer to your question is tooth decay.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.