[单选题]
A microscope has an objectivb8u x;y ;:gigye leni9x6 li u(yor*i3:tezs of focal length 1.4 mm and an eyepiece of focal length 20 mm, adjusted for minimum eyestrain. A blood sample is placx*(6 r l:y3 oueti9iized 1.5 mm from the objective. How far apart are the lenses?
A. $20 \; mm$
B. $21 \; mm$
C. $23 \; mm$
D. $41 \; mm$
