[单选题]
A compound microscope has a 18-cm barrel and tkw-cjm 2 f;n9i+kyl;an objective wivrtqt-6mkees ozl v9 2*5tc-5:xxd 6b th a focal length of 8.0 mm. eld k9vt:2qxz cr6m6--x btve*s5o5 t 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. $2.3 \; cm$
