[单选题]
A DC motor of internay pe;8t2y+rjt l i:5*p+ rt xfu9kte1fc w m06znresistance 6.0 W is connected to a 24-V power supply. The opknxe :51 wutcmr0*fi 69fpzt+erating current 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. $24 \; V$
