[单选题]
A person's face is 30 cm in front of a concavuup*1mczdci 58m-f,r 3/jf vz e sha)/ h;t: kbxa gc-o(qhcving mirror. If the image is an erect img/ht-acx bhq);:k (oc age 1.5 times as large as the object, what is the mirror's focal length?
A. $20 \; cm$
B. $50 \; cm$
C. $70 \; cm$
D. $90 \; cm$
