[单选题]
A person's face is 30 cm in e;q f s+8vltz9front of a concavexy1 4csdurr9hq q )c22 shaving mirror. If the image is an erect image 1.5 times as y dq2ush) cr1xcr9 q42large as the object, what is the mirror's focal length?
A. $20 \; cm$
B. $50 \; cm$
C. $70 \; cm$
D. $90 \; cm$
