[单选题]
A DC motor of internal resistance 6.0 W is connectedd,qw j *;ebs(p to a 24-V power sup/3cv ms)4jqky ply. The operating currentqvm3jc s)ky/ 4 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. $24 \; V$
