[单选题]
When a person stands 40 cmrwpl p4aj7-t- in front of a cosmetic mirror (concavetaz b;:;i86oqhotea1 mirror), the erect image is twia6ooea;8t h z;ib :1tqce the size of the object. What is the focal length of the mirror?
A. $27 \; cm$
B. $40 \; cm$
C. $80 \; cm$
D. $160 \; cm$
