Assertion (A): Sixth and seventh periods in the periodic table contains 14 elements.
Reason (R): In the periodic table, 14 elements of sixth and seventh periods are known as lanthanoids and actinoids.

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that the sixth and seventh periods of the periodic table contain 14 elements each. We need to evaluate if this statement is true or false.

2. **Analyzing the Periods**: In the periodic table, the sixth and seventh periods actually contain a total of 32 elements each. This includes various blocks of elements (s, p, d, and f blocks).

3. **Identifying the Elements**:
- The sixth period consists of:
- 2 elements from the s-block (Group 1 and 2)
- 10 elements from the d-block (transition metals)
- 14 elements from the f-block (lanthanoids)
- 6 elements from the p-block
- The seventh period also consists of:
- 2 elements from the s-block
- 10 elements from the d-block
- 14 elements from the f-block (actinoids)
- 6 elements from the p-block

4. **Conclusion on Assertion**: Since both the sixth and seventh periods contain 32 elements, the assertion is **false**.

5. **Understanding the Reason**: The reason states that the 14 elements of the sixth and seventh periods are known as lanthanoids and actinoids, respectively. This statement is **true** because:
- The 14 f-block elements in the sixth period are indeed called lanthanoids.
- The 14 f-block elements in the seventh period are called actinoids.

6. **Final Evaluation**: Since the assertion is false and the reason is true, the overall statement (Assertion and Reason) is incorrect.

**Final Answer**: Assertion (A) is false, and Reason (R) is true.

---