【单选题】As a general rule, the first troubleshooting action to be taken in checking faulty electric control devices is to ______. prefix="o" ns="urn:schemas-microsoft-com:office:office" ?xml:namespace>
A.
draw a one line diagram of the circuitry
B.
test all fuses and measure the line voltage prefix="o" ns="urn:schemas-microsoft-com:office:office" ?xml:namespace>
C.
take megger readings
D.
insulate the apparatus from ground prefix="o" ns="urn:schemas-microsoft-com:office:office" ?xml:namespace>