[单选题]
A microscope has an objeer8 5dbgv v:0zctivex6wvv4mr;ek( bpgc68 lens of focal length 1.4 mm and an eyepiece of focal l6e vv pm;w cxr68(4bkgength 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. $41 \; mm$