[单选题]
A compound microscope has* ,x+uwgy6tj v a 18-cm barrel;d.ajtqhf0 g18((xramnf1d 1u fxh u7 and an objective with a focal length of 8.0 mm. What is the focal length of the eyepiece to give a total magnification of 2q.hjht1 adxmd;u fg(171(n a0urff 8x40?
A. $0.13 \; cm$
B. $1.5 \; cm$
C. $1.9 \; cm$
D. $2.3 \; cm$
