The circuit shown in the figure has 4 boxes each described by inputs P, Q, R and outputs Y, Z with $${\text{Y}} = {\text{P}} \oplus {\text{Q}} \oplus {\text{R,}}\,\,{\text{Z}} = {\text{RQ}} + \overline {\text{P}} {\text{R}} + {\text{Q}}\overline {\text{P}} $$
The circuit acts as a

A. 4 bit adder giving P + Q

B. 4 bit subtractor giving P - Q

C. 4 bit subtractor giving Q - P

D. 4 bit adder giving P + Q + R

Answer: Option B