Use Interfaces when…

- You see that something in your design will change frequently.

- If various implementations only share method signatures then it is better to use Interfaces.

- You need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.

Use Abstract Class when…

- If various implementations are of the same kind and use common behavior or status then abstract class is better to use.

- When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.

- Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

Basically India's unemployment is structural in nature. It is associated with the inadequacy of the productive capacity to create enough jobs for all those able and willing in india. Not only is the productive capacity  (largely capital stock) much below the needed quantity, it is also found to be increasing at a slow rate. As against this, additions to labour force are being made at a fast rate on account of rapidly rising population. Thus, the absolute level of unemployed persons is rising every year. 

Income easticity of demand measures the responsiveness of demand for any goods to changes in income. With increase in incomes generally the demand for farm products like rice wheat etc, will not change drastically. With income increase, demand for luxuries such as consumer durables etc will increase. The income elasticity  of demand for farm products is thus low.

Saw dust contains carbon which is oxidised to carbon  dioxide by concentrated nitric acid;

Parabola, because when the coin is dropped at a particular point, it has the same velocity as the running train. So, by Newton's first law, it continues its motion horizontally with that velocity. But then, it is also pulled down by earth's gravity continuously. In this situation its horizontal motion gives way to the parabolic motion until it reaches the ground.

Black  surfaces are good emitters of heat ( They are also good absorbers of heat). If the black surface is rough too, the emitting will be better. So (b) is the correct answer. Shining surfaces reflect back radiation. So if the black surface is also shining the effectiveness of emitting heat is lessened.

Before transferring the fish, the position is the man with the fish and the bucket of water. After transferring the position is: the man without the fish, but the fish inside the water in the bucket. Thus to know  whether there is any change in weight, it is enough to consider whether there is any change in weight when the fish in air is transferred into the water. The answer is there is no change as when in water though it loses weight due to upthrust of water on it, the level of water increases and exerts an equal force at the bottom.So, (c) is correct.

The weight of the man mg is acting on the floor. As the lift is moving up with an acceleration, say a, the man too in it is having this acceleration. The force producing this acceleration is ma. By Newton's third law, this force will have an equal reaction pressing the floor equal to ma. Therefore the total resultant reaction on the floor is mg+ma = m(g+a) which is mg, the weight of the man. So (a) is correct.