[单选题]
A spherical concave mirror habj) dh+wte,lao/ fq(7:/vdd h ctc65m s a r q.i zhczdqq( )9 pym+xeey6z-k5+ e1vadius of curvature of 20 cm. How far from the mirror is(ze5qp+y9 vdz.-meh 6ie)x+qqky 1cz the focal point located?
A. $10 \; cm$
B. $20 \; cm$
C. $30 \; cm$
D. $40 \; cm$
