A programmable logic array (PLA) is shown in the figure. The Boolean function F implemented is

Figure I shows a 4-bits ripple carry adder realized using full adders and Figure II shows the circuit of a full-adder (FA). The propagation delay of the XOR, AND and OR gates in Figure II are 20 ns, 15 ns and 10 ns respectively. Assume all the inputs to the 4-bit adder are initially reset to 0. At t=0, the inputs to the 4-bit adder are changed to $${X_3}$$$${X_2}$$$${X_1}$$$${X_0}$$ =1100, $${Y_3}$$$${Y_2}$$$${Y_1}$$$${Y_0}$$ = 0100 and $${Z_0}$$=1. The output of the ripple carry adder will be stable at t (in ns) = ____

The functionality implemented by the circuit below is

Identify the circuit below.