Assertion: The units of some physical quantities can be expressed as combination of the base units. ltbr. Reason: We need only a limited number of units for expressing the derived physical quantities.

Options

A. if both assertion and reason are true reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation fo assertion.
C. If assertion is true but reaso is false.
D. IF both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that "The units of some physical quantities can be expressed as a combination of the base units."

- **Explanation**: This means that certain physical quantities, which are derived quantities, can be represented using the fundamental or base units. For example, the unit of velocity is expressed as meters per second (m/s), where "meter" is a base unit for distance and "second" is a base unit for time.

### Step 2: Understand the Reason
The reason states that "We need only a limited number of units for expressing the derived physical quantities."

- **Explanation**: This implies that to express various derived quantities, we do not require an infinite number of units. Instead, a small set of base units (like meter, kilogram, second, etc.) can be combined in different ways to derive other units. For instance, velocity (m/s) and acceleration (m/s²) can be derived using the same base units.

### Step 3: Analyze the Relationship
Now we need to determine if the reason correctly explains the assertion.

- Both statements are true. The assertion is correct because derived units can indeed be expressed as combinations of base units. The reason is also correct because it highlights that only a limited number of base units are needed to express a wide variety of derived quantities.

### Step 4: Conclusion
Since both the assertion and reason are true, and the reason provides a correct explanation for the assertion, we conclude that:

- **Final Answer**: Both assertion and reason are true, and the reason is a correct explanation of the assertion.