[单选题]
A DC motor of internal resistance 6.0 W is connected togrns5mbvt c */* qj;v- a 24-d xt(ax*9epg +eeg5.c V power supply. The operating current is 1.0 A at full speed5excp.de*+x9eg a t (g. 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. $24 \; V$
