[单选题]
A compound microscope has a 18-cm barrel and an objective with a focwq7tk)fxpk/ e4*8jr5fz j f+r al lengh- ald:9rfkckwkor y 7tzq2 d+u 53(.hth of 8.0 mmkl+u-o 3tcqky(wzd5k 7h9:fdrh2r. a . What is the focal length of the eyepiece to give a total magnification of 240?
A. $0.13 \; cm$
B. $1.5 \; cm$
C. $1.9 \; cm$
D. $2.3 \; cm$
