[单选题]
A microscope has an objevl wb .vf x8.lcg9cm;/tkuv3w2 +hja:x9fw,c brdg*dx fyy7cl t;*p0 :h5w+l ctive lens of focal length 1.4 mm and an eyepiece of focal letydfc+gl:,;p *bc0h9xf7l*x5 wwrd yngth 20 mm, adjusted for minimum eyestrain. A blood sample is placed 1.5 mm from the objective. How far apart are the lenses?
A. $20 \; mm$
B. $21 \; mm$
C. $23 \; mm$
D. $41 \; mm$
Post time 2024-1-26 22:03:25