[单选题]
A DC motor of internal resf76p v2hg+/t4ty2o oa hv8tf aistance 6.0 W is connected to a 24-V pow ymhs1j7 v1a2hyf1/bcer supply. The operating current y71 c11hs/fhabm2yjv is 1.0 A at full speed. 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. $24 \; V$
