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: Radiology
Measurement used to estimate gestational age in first trimester. 
Select and explain. 
A. CRL, B. Femur length, C. BPD, D. Head circumference
A: CRL
Answer: A: CRL 
Explanation: Crown - rump length - 1st trimeter.
FL - 2nd and 4rd trimeter.
AC and HC  - 3rd trimester.