Two square waves of equal period T, but with a time delay τ are applied to a digital circuit whose truth table is shown in the following figure.

X

Y

Output

0

0

1

0

1

0

1

0

0

1

1

1

The high and the low levels of the output of the digital circuit are 5 V and 0 V, respectively. Which one of the following figures shows the correct variation of the average value of the output voltage as a function of τ for 0 ≤ t ≤ (T/2)?