[单选题]
A DC motor of internal resistance 6.0 W is concj+ 9wu4isqn) nected to al9s2 iv:wsdpoa1am+ l y0g +.f 24-V power supply. The op+l9o vy1 mai .:ssfa+pd2wl0gerating 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. $24 \; V$
