[单选题]
A person's face is 30 cm iuzm4x9, y m,ty604mtcwwx dr3 n f)bzetvych:1. ront of a concave shaving mirror. If the image is an erect image 1.5e1.zybv )t:h c times as large as the object, what is the mirror's focal length?
A. $20 \; cm$
B. $50 \; cm$
C. $70 \; cm$
D. $90 \; cm$
