[单选题]
When a person stands 8rit4wep8 5dgv6oy)d 40 cm in fry6l 1zihq 5,ekont of a cosmetic mirror (concave mirror), the erect image is twice the size of the object. Wh h,65iyzqe l1kat is the focal length of the mirror?
A. $27 \; cm$
B. $40 \; cm$
C. $80 \; cm$
D. $160 \; cm$
