advance diode question( homework i dont understand )

[07potatoes36]

in the attachment there are four circuit diagram (A,B,C,D)
questions:
1). What are the condition for the diode to conduct in circuit A,circuit B,circuit C , circuit D?
example : in A the VRA must be bigger than VA for the diode to conduct.



please help me , i know that diode only allows current to flow in one direction.. but the problem is there are resistor and another voltage source connected here ... please help me..