Answer:
A Direct Current (DC) electromagnetic field refers to a constant or static DC electric or DC magnetic field emission which has a frequency of 0 Hz. DC magnetic fields can also be created from a DC power source of electricity such as battery power, solar power, and wind power.
The answer to this question is letter "A. cite a source". Citing a source is an important content of the presentation so as not to take the credit away from the real source of the ideas presented. Not citing the sources could be a ground for plagiarism.