This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscles of Lower Limb, Subject name: Anatomy
Chief Extensor at knee joint is 
Select and explain. 
A. Biceps femoris, B. Quadriceps femoris, C. Semitendinosus, D. Semimembranosus
B: Quadriceps femoris
Answer: B: Quadriceps femoris 
Explanation: Quadriceps femoris is knee Extensor. Hamstrings (Biceps femoris long head, Semitendinosus, Semimembranosus and Adductor magnus) are Knee Flexors.