(A) : Invar is used in metal tapes, balance wheels and pendulum clocks.
( R) : The coefficient of linear expansion of invar is very low.

Options

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

Answer

To solve the question regarding the assertion and reason about Invar, we will break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "Invar is used in metal tapes, balance wheels, and pendulum clocks."

- **Explanation**: Invar is a nickel-iron alloy known for its unique properties, particularly its low coefficient of thermal expansion. This means that it does not expand or contract significantly with temperature changes, making it ideal for precision instruments.

### Step 2: Understand the Reason
The reason states that "The coefficient of linear expansion of Invar is very low."

- **Explanation**: The coefficient of linear expansion is a measure of how much a material expands per degree of temperature increase. Invar has a very low coefficient of linear expansion, which means it maintains its dimensions very well across a range of temperatures.

### Step 3: Connect the Assertion and Reason
Now we need to establish the relationship between the assertion and the reason.

- **Explanation**: The reason given (low coefficient of linear expansion) directly supports the assertion (use of Invar in precision instruments). Because Invar does not expand or contract much with temperature changes, it maintains accuracy in measurements and timing in devices like metal tapes, balance wheels, and pendulum clocks.

### Step 4: Conclusion
Both the assertion and the reason are true, and the reason is a correct explanation for the assertion.

- **Final Answer**: Both the assertion (A) and the reason (R) are correct, and R is the correct explanation of A.