Coal is a combustible organic rock which is primarily made of carbon, nitrogen and oxygen. It is mainly classified into different ranks or grades, based on the percentage of carbon in it.

There are more than 300 breeds of dogs in the world. These breeds are usually classified according to the type of job performed by them.

Clouds are generally classified into three types – High, Low and Middle types, depending upon their region of formation. They are further classified according to their shape and size.

Animal kingdom consists of many different types of animals, but still all the animal share common characteristics. They all are multicellular means made up of more than one cell. They do not contain any cell walls like plant cells, and they consume or eat other organisms too.

Diodes are important two terminal devices which are used in many electrical components. They fall into the group of passive electrical devices. Generally, they allow flow of current in only one direction and try to prevent the flow of current in the opposite direction. There are many different types of diodes, based on their specific characteristics and applications.

The North and South poles are Earth’s ending poles. The North Pole is the northernmost point of the Earth's axis; commonly known as the Arctic region. While, the South Pole is the southernmost point of the Earth's axis; commonly known as the Antarctic continent.

India and Pakistan are two countries lying in the south of Asia. India and Pakistan are both neighbors and share the same historical events before their Independence. On the other hand, they differ in many aspects like area, primary language, dominant religion, etc.

Asynchronous and Synchronous are two different methods of transmission synchronization. The major difference between them lies in their transmission methods, i.e. Synchronous transmissions are synchronized by an external clock; whereas Asynchronous transmissions are synchronized by special signals along the transmission medium.

Insulator is a material which is not able to conduct electricity. On the other hand, dielectrics are insulators which polarized when an electrical field is applied.

Mixture and compound are two different types of matter. Individual substances are joined chemically in a compound. On the other hand, the individual substances comprising a mixture are not joined chemically with each other.

Jaundice is a disease caused due to liver malfunctioning; while, Hepatitis are types of viruses which causes the liver infections and other diseases. In Jaundice a person’s skin color changes to yellow due to the excess bilirubin concentration in the bloodstreams; while, Hepatitis are types of viruses which causes liver diseases and further may also become a cause of jaundice.

Ill and Sick are two words of similar meaning generally used to refer the unwell or not desirable health state of an individual. Basically, the word ‘sick’ is an informal term used to refer the short-term diseases or ailments; while, the word ‘ill’ is more formally used to refer any long or short-term diseases or ailments.

Mahogany is a dark red brown colored tropical wood indigenous to South America, Africa and Central America. Teak is a close grained hardwood which is yellow-to dark brown in color. It is indigenous to Indonesia, India and Central America.