[单选题]
A person's face is 30 1a5 6fmanb48g4t,xejx ckp n9 cm in front of a concave shavingh k/ev.mc j12g mirror. If the image is an erect image 1.5 tmh.ge 1vkc2 /jimes as large as the object, what is the mirror's focal length?
A. $20 \; cm$
B. $50 \; cm$
C. $70 \; cm$
D. $90 \; cm$
