A full wave rectifier circuit along with the input and output voltages is shown in the figure
The contribution to output voltage from diode 2 is if in first half cycle diode 1 is in forward biased condition.
In the positive half cycle of input ac signal diode D1 is forward biased and D2 is reverse biased so in the output voltage signal, A and C are due to D1. In negative half cycle of Input ac signal D2 conducts, hence output signals B and D are due to D2.