[单选题]
A object is 12 cm in front of a concave mirror, and taencxj/*m;1iz 8swj 7t,hn x9 he image 3/bl n1i)glc+ (gpni , lr+h; qwxkyi1is 3.0 cm in front of the mirror. What is the flb1,/( i g1h)xlcq +l pk wii+3grnn;yocal length of the mirror?
A. $15 \; cm$
B. $4.0 \; cm$
C. $2.4 \; cm$
D. $1.3 \; cm$
