Answer:
A. Motherboard has failed.
C. Processor has gone bad or is not seated properly.
Explanation:
The computer system is a digital or electronic device, which comprises of an input device, output device, memory and storage device and a processor, all controlled by a software interface known as the operating system.
The hardware and software components are all integrated on a printed board known as a motherboard.
When the motherboard is faulty, the system looses it functionality and the screen stays off but the power supply indicator light stays on. This is also the same for the processor.