[单选题]
A compound microscop /9:csrrg a7ihe has a 18-cm barrel and an objective with a foca u-aerzle: k7/l length of 8.0 mm. What is the focal length of the eyepiece to give a total magnificael/kz- 7 :aerution of 240?
A. $0.13 \; cm$
B. $1.5 \; cm$
C. $1.9 \; cm$
D. $2.3 \; cm$
