question from A+ pract test

[kapa0016]

Hi, I am doing a practise test for A+ and there is one question that i have a hard time to see the importance between possible answeres. Maybe you can give me tell me why that answer and not that...seeem to bee all good.
Q. You are installing a computer for inventory control at a warehouse. Three serial devices must work on the same computer: a mouse, a modem, and a serial barcode scanner. Which of the following are valid configuration?(select two)
A) COM 1=serial mouse, COM 2=serial barcode reader, COM3 = Modem, COM4=open
B)COM 1=serial mouse, COM 2=serial barcode reader, COM3 = open, COM4=Modem
C)COM 1=serial mouse, COM 2=open, COM3 = serial barcode reader, COM4=MOdem
D)COM 1=serial mouse, COM 2=Modem, COM3 = open, COM4=serial barcode reader.
Answ(B&D)

[rond36]

The answer is "B" or "D" depending on the type of modem that is to be installed

"A" can't be correct because COM1 and COM3 share an IRQ and DMA and would cause a conflict between the mouse and modem.

"B" would be correct if the modem is an internal win-modem that installs a virtual COM4 and can use that port. (most software Modems can only use virtual COM4)

"C" can't be correct because COM1 and COM3 share an IRQ and DMA and would cause a conflict between the mouse and barcode reader.

"D" would be correct if the modem is an external hardware modem and the barcode reader software is capable of installing a virtual COM4 and can use it.

[classicsoftware]

If B&D are the correct answers it is because the COM1 and COM3 share the same IRQ and the mouse will not play nice with another device sharing the same IRQ, while the other two devices will be able to share an IRQ.