[单选题]
A concave mirror with a ( r3g 6x yb+r*dphs)azradius of 20 cm creates a real ima rv u+uu+.qnw*,/p.ta ;fdah age 30 cm from the mirror. *wpa +n+;fata q. .huv/u,druWhat is the object distance?
A. $20 \; cm$
B. $15 \; cm$
C. $7.5 \; cm$
D. $5.0 \; cm$
