Which groups attacked the Song Dynasty from the north, contributing to military strain and vulnerability?

Which groups attacked the Song Dynasty from the north, contributing to military strain and vulnerability?

• A. The Western Turks and Sogdians with economic sanctions.
• B. No significant northern threats during the period
• C. Only the Mongols with overwhelming cavalry
• D. Liao, Jin, and later the Mongols; defense costs and military weakness contributed to vulnerability.

Answer: (D)