This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ULTRASOUND, Subject name: Radiology
Which mode of ultrasound help is Orbital biometry? 
Select and explain. 
A. A- mode, B. B-mode, C. M-mode, D. D-mode
A: A- mode
Answer: A: A- mode 
Explanation: A Mode Ultrasound is Amplitude Mode used for measuring Axial length of Eyeball and Orbital Biometry. A scan in Opthalmology is the A Mode Ultrasound. B mode is used in routine ultrasonography. M mode is used in chest and obstetric USG. D mode is used for vessels.