This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Anatomy
Earliest of fetal hea can be detected at: 
Select and explain. 
A. 6-6.5 weeks, B. 6.5-7 weeks, C. 7.1-7.5 weeks, D. 8 weeks
A: 6-6.5 weeks
Answer: A: 6-6.5 weeks 
Explanation: When the question says - Cardiac activity is earliest detected at it means it's about TVS (Transvaginal sonography),which is seen at 6 weeks. Ref: Callen 4e pg 119,120.