This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Pathology
Focal length of high power objective lens of microscope? 
Select and explain. 
A. 40mm, B. 16mm, C. 20mm, D. 4mm
D: 4mm
Answer: D: 4mm 
Explanation: Ans. is 'd' i.e., 4 mmProperties of Microscopic Objective LensPropertyScanningLow PowerHigh PowerOil ImmersionMagnification4x10x40-45x 90-100xNumerical Aperature0-100.250.55-0.651.25-1.4Approximate focal length40 mm16mm4 mm 1.8-2.0 mmWorking distance17-20 mm4-8 mm0.5-0.7 mm0.1 mmApproximate resolving power with light of 450 nm(blue light)2.3 mm0.9 mm0.35 mm0.18 mm