[单选题]
A DC motor of internal resistance 6.0 W is connectedf1a tq 7 q))5ke1itcht2btk)cjug; n4 to a 24-V power supa (cffdh4 mn9,zwa8k,ply. The operating current is 1.0 A at full speed. What isk zmf8,d ah(na,f wc94 the back $emf$ when the motor is running at half speed?
A. $6.0 \; V$
B. $9.0 \; V$
C. $18 \; V$
D. $24 \; V$
