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: Physiology
Estrogen is secreted by ? 
Select and explain. 
A. Granulosa cells, B. Theca luteal cells, C. Theca interna, D. Theca externa
A: Granulosa cells
Answer: A: Granulosa cells 
Explanation: Ans. is 'a' i.e., Granulosa cells