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In this problem, we are given a pyramid \( SABC \) with a parallelogram base \( ABCD \). Points \( M \) and \( N \) are midpoints of \( SC \) and \( SD \), respectively. We are tasked with identifying the correct statement about the line \( MN \) and its parallelism with one of the planes.

### Step-by-Step Solution:
1. **Pyramid Structure**: The base \( ABCD \) is a parallelogram, and we have edges \( SA, SB, SC, SD \) extending from the apex \( S \) to the vertices of the parallelogram.

2. **Midpoints of Edges**: Since \( M \) is the midpoint of \( SC \) and \( N \) is the midpoint of \( SD \), the line \( MN \) is the midline of the triangle \( SCD \). 

3. **Midline Property**: In any triangle, the midline joining the midpoints of two sides is parallel to the third side. Therefore, in the triangle \( SCD \), the line \( MN \) is parallel to the base \( CD \).

4. **Plane Parallelism**: The plane that contains \( CD \) is \( (SCD) \). Since \( MN \parallel CD \), it follows that \( MN \parallel (SCD) \).

Thus, the correct answer is **B. \( MN \parallel (SCD) \)**.

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### Related Questions:
1. How does the midline theorem apply in this case?
2. Can you explain how to determine parallelism between a line and a plane?
3. How would this problem change if the base of the pyramid was not a parallelogram?
4. How does the structure of a parallelogram influence the problem?
5. What other methods could be used to determine the correct parallel plane?

**Tip**: Remember that a line parallel to one side of a triangle is always parallel to the entire plane containing that side!

Cho hình chóp SABC có đáy là hình bình hành. M, N lần lượt là trung điểm của SC và SD. Mệnh đề nào sau đây là đúng?

Analyze the geometry of a pyramid with a parallelogram base to determine the correct statement about the parallelism of line MN, which joins the midpoints of SC and SD. This problem explores midline theorem and plane geometry, suitable for high school students.

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