The present century has witnessed remakable increase in the population of the world and specially in India. One major factor for this is that

Options

A. many children per family begin to reach the reproductive age
B. more childen are born in each family
C. many people are marrying in younger age group
D. older people have begin to live longer .

Explanation

To solve the question regarding the remarkable increase in the population of the world, especially in India, let's analyze the options provided step by step:

### Step 1: Understand the Context
The question states that there has been a significant increase in the population in the present century, particularly in India. We need to identify the major factor contributing to this increase.

### Step 2: Analyze Each Option
1. **Many children per family begin to reach the reproductive age**:
- While this could contribute to population growth, it does not explain the overall increase in population as a primary factor. The number of children per family has been decreasing in many regions, including India.

2. **More children are born in each family**:
- This option suggests that families are having more children. However, in recent decades, there has been a trend towards smaller family sizes due to various socio-economic factors. Thus, this is not a major factor.

3. **Many people are marrying in younger age group**:
- The trend of marrying at younger ages has not been consistent. In fact, many studies show that the average age of marriage has been increasing. Therefore, this option is also not a significant factor.

4. **Older people have begun to live longer**:
- This option highlights the increase in life expectancy due to advancements in healthcare, nutrition, and overall living conditions. As older individuals live longer, the population increases because the mortality rate decreases while the birth rate may not necessarily increase.

### Step 3: Conclusion
After analyzing all the options, the most significant factor contributing to the remarkable increase in population, particularly in India, is that **older people have begun to live longer**. This is due to improved healthcare and living standards, resulting in lower mortality rates.

### Final Answer
The correct answer is: **Older people have begun to live longer.**

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