This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Flexor of lumbar spine is/are -a) Erector spineb) External oblique musclec) Internal oblique muscled) Rectus abdominise) Psoas major 
Select and explain. 
A. ad, B. be, C. ace, D. de
D: de
Answer: D: de 
Explanation: Rectus abdominis, psoas major and gravity are principal flexors of thoracolumbar vertebrae.