已知线段 AB = 20 cm, C 为线段 AB 上一点, 且 AC = 4 cm. 若 M, N 分别是 AC, BC 的中点, 则 MN = ( ) A. 13 cm B. 12 cm C. 10 cm 或 8 cm D. 10 cm---**Question Information:** 例 1 (母题)(人教七上 P128 练习第 3 题改编) **Question Stem:** 如图 1-1-1, 已知线段 AB = 20 cm, C 为线段 AB 上一点, 且 AC = 4 cm. 若 M, N 分别是 AC, BC 的中点, 则 MN = ( ) **Diagram Description:** Type: Line segment diagram. Main Elements: A horizontal line segment is shown with points A, M, C, N, B labeled in order from left to right. Point M is between A and C. Point N is between C and B. Text label "图 1-1-1" is below the line segment. **Options:** A. 13 cm B. 12 cm C. 10 cm 或 8 cm D. 10 cm