$(20$ Points$)$ True or False

a$)$ When using the superposition theorem, each ideal voltage and current source must be replaced by an open circuit.

b$)$ Thevenin's theorem permits the reduction of any two $-$terminal linear dc network to one having a single voltage source and a series resistor.

c$)$ Norton's theorem permits the reduction of any two$-$terminal linear dc network to one having a single current source and a series resistance.

d$)$ Under maximum power transfer conditions, the operating efficiency of a system is $\backslash (100\backslash \%\backslash ).$