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: Gynaecology & Obstetrics
Cardiac activity of the fetus seen by ultrasound by: 
Select and explain. 
A. 4 weeks, B. 5 weeks, C. 6 weeks, D. 7 weeks
C: 6 weeks
Answer: C: 6 weeks 
Explanation: 6 weeks