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 motion is visible with trans vaginal imaging  when the embryo length has reached 
Select and explain. 
A. 5 mm, B. 10 mm, C. 15 mm, D. 20 mm
A: 5 mm
Answer: A: 5 mm 
Explanation: Cardiac motion is visible when embryo reaches length of 5mm.