Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Hydrogen is the most abundatn element in the Universe, but it is not the most abundant gas in the troposphere .
Reason R : Hydrogen is the lightest element . In the light of the above statements , choose the correct answer from the options given below :

Options

A. A is true but R is false
B. Both A and R are true and R is the correct explanation of A
C. A is false but R is true
D. Both A and R are true but R is NOT the correct explanation of A

Answer

To solve the question, we need to analyze the two statements provided:

**Assertion A:** Hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere.
**Reason R:** Hydrogen is the lightest element.

### Step 1: Evaluate Assertion A
- **Hydrogen's Abundance in the Universe:** It is a well-known fact that hydrogen is the most abundant element in the universe, making up about 75% of its elemental mass.
- **Hydrogen's Presence in the Troposphere:** In the Earth's troposphere, the most abundant gas is nitrogen, which constitutes about 78% of the atmosphere. Hydrogen, being very light, is present in trace amounts and does not accumulate in the troposphere.

**Conclusion for Assertion A:** The assertion is **true**.

### Step 2: Evaluate Reason R
- **Hydrogen as the Lightest Element:** Hydrogen is indeed the lightest element, consisting of only one proton and one electron, and it does not have any neutrons. This property contributes to its low density and high tendency to escape into space.

**Conclusion for Reason R:** The reason is also **true**.

### Step 3: Analyze the Relationship Between Assertion A and Reason R
- **Explanation Link:** The reason provided (that hydrogen is the lightest element) explains why hydrogen is not abundant in the troposphere. Its lightness allows it to escape the gravitational pull of the Earth more easily than heavier gases, leading to its lower concentration in the troposphere.

### Final Conclusion
Both Assertion A and Reason R are true, and Reason R correctly explains Assertion A. Thus, the correct answer is that both statements are true and the reason is the correct explanation of the assertion.

### Answer: Both Assertion A and Reason R are correct, and Reason R is the correct explanation for Assertion A.

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