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Secondary pollutants include ozone, which is formed when hydrocarbons (HC) and nitrogen oxides (NOx) combine in the presence of sunlight.

Whereas Primary pollutant are those pollutants that are directly emitted from their sources, like CO2, CO, SO2 as these gases are directly emitted form burning of fossil fuels.

Acid rain, which is formed when sulfur dioxide or nitrogen oxides react with water, sulfuric acid etc.

Hence, Acidrain is a secondary air pollutant.

The process of conversion of milk into curd is called fermentation.  It is a chemical change. Milk consists of globular proteins called casein. As the microorganism lactobacillus react with milk and turn it to curd. For it 37 degree temperature is needed. But sometimes when it does not get favourable condition, then other microorganism react and makes the curd taste different.

Economic resources are those goods and services that can be converted into valuable products for individual and business purposes.

There are four major types of economic resources:

1. land,

2. labor,

3. capital and

4. entrepreneurship.

Land and labor are natural and human resources, while capital means physical resources or money.

Entrepreneurship converts these resources into usable goods.

However, resources have finite values, and proper utilization depends on economic systems and economic decisions.

An icycle, a hanging spike of ice formed by the freezing of dripping water forms due to cool in winter and dies in summer due to hot.

While eating a watermelon, we all go along the red and stops at green.

A rowboat can be put into motion by using oar. Similaarly, a skateboard can be put into motion by using foot.

The other options like walk and running are not objects to be in motion. And loafers are something worn on foot.

Hence, Oar : Rowboat :: Foot : Skateboard.