In which category is the IEC 60309 an industry standard?

A.
Power

B.
Airflow

C.
Humidity

D.
Space

Explanation:
IEC 60309 is an international standard from the International Electrotechnical Commission (IEC) for “plugs,
socket-outlets and couplers for industrial purposes”.
The cable connectors and sockets are colour-coded, according to the voltage range and frequency used; for
example, common colours are yellow for 100–130 volts and 50–60 Hz, blue for 200–250 volts and 50–60 Hz,
and red for 400–480 volts and 50–60 Hz. Color codes depend on the highest, phase-to-phase, voltage.
https://en.wikipedia.org/wiki/IEC_60309