[单选题]
A DC motor of internal r/*(bc a9foo/0plec u4w+8ddwy vm;bmesistance 6.0 W isd l nf.6b73xj2wog9xr connected to a 24-V power supply. The operating current is 1.0 A at full speed3wx26og j7l9ndr xb.f . What is the back $emf$ when the motor is running at half speed?
A. $6.0 \; V$
B. $9.0 \; V$
C. $18 \; V$
D. $24 \; V$
