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: ENT
Which focal length in the objective piece of microscope is commonly used for ear surgery? 
Select and explain. 
A. 100mm, B. 250 mm, C. 450mm, D. 950 mm
B: 250 mm
Answer: B: 250 mm 
Explanation: Focal length of objective lens:

For ear surgery - 200 mm/ 250 mm
For Nose / Paranasal sinus surgery - 300 mn
For Laryngeal surgery-400 mn