[单选题]
A person's face is 30*sdqwz 3uzzo 87+ +6khbvrrnhw0 +j1y cm in .rl*etmku1 lt+d1)q dfront of a concave shaving mirror. If the image is an erect image 1.5 times as large as the object, 1k r1.d)*utql mdt+lewhat is the mirror's focal length?
A. $20 \; cm$
B. $50 \; cm$
C. $70 \; cm$
D. $90 \; cm$
