[单选题]
A compound microscope has a 18-cm barr*r uznuq(ff-:el and an objective with a focal length opda0n gvfdg;7;t4zr r3t q;r 1f 8.0 mm. What is the focal length of the eyepiece to giva;d 3gdrr;nvt1tgqfrz ;70 p4 e a total magnification of 240?
A. $0.13 \; cm$
B. $1.5 \; cm$
C. $1.9 \; cm$
D. $2.3 \; cm$