$2.$ Arithmetic circuits $(8$ points$)$

See the $4-$bit adder above and answer the following questions.

a$.$ What is the truth table for one bit Full Adder? $($FA$)(\backslash (4^\{\backslash$prime$\}\backslash ))$

$10$

b$.$ How does this circuit achieve subtraction? $(\backslash (2^\{\backslash$prime$\}\backslash ))$

c$.$ What is the worst$-$case propagation delay in terms of $\backslash ($ n $\backslash )$ when you use the adder to imolement a n$-$bit addition? $(\backslash (2^\{\backslash$prime$\}\backslash ))$