Cow dung and vegetable waste are decomposed in the absence of oxygen to give gobar gas, also called biogas.

The energy gained by an object due to change in its shape is potential energy. Hence, option (1) is correct.

The quality of a tone depends upon the overtones that are present there.

Radar is a device used for detecting distant objects.

The maximum displacement of a vibrating body from its rest or mean position is called its amplitude.

Human beings cannot hear sounds of frequencies below 20 Hz.

The sky appears blue because the Earth`s atmosphere scatters blue light the most.

A red cross on a white background, when seen through a red filter, will become invisible.

The working of a dynamo is based on the principle of electromagnetic induction.

Work done is mgh. Here, we know the weight or mg, so we need to know the height of the table where the doll is placed in order to calculate work done.

Whenever two different materials rub against each other, it is likely that one will leave with more electrons than it started with. And since electrons are negatively charged, so when a body is negatively charged by friction, it means the body has acquired excess of electrons.

The condition for an electric charge to flow from one point to another is that the two points must have electric potential difference.

The line passing through the pole and the centre of curvature of a spherical mirror is called its principal axis.

We know V = IR, where V is the voltage, I is the current and R is the resistance. I is inversely proportional to R. So, if the resistance of a conductor is increased, current will decrease.

Correct option is (4).

Correct option is (3).

Light year is a unit of distance.

Solar energy cannot be used on a cloudy day as it requires sunlight for its operation.

Radiation is the form of heat transfer that can occur in the absence of a medium. Thus, it is possible to transfer heat in vacuum through the process of radiation.

The wind speed should be more than 15 km/hr to maintain the required speed of the turbine.