[单选题]
A object is 12 cm in front of a concave mirraiw0qvs/ d/ p6s bgm)eeru27i:x 5x7nor, and the image is 3.0 cm in fronr5r1(j(n o2 xvlhnyi ;t ;ynx rnij(ov (21h5rl of the mirror. What is the focal length of the mirror?
A. $15 \; cm$
B. $4.0 \; cm$
C. $2.4 \; cm$
D. $1.3 \; cm$
