(a) Explain why, given the possibility of source transformation, Norton's theorem can be considered as a simple consequence of Thevenin's theorem. (b) Explain why the short-circuit method for determining the Thevenin resistance of a circuit cannot be applied if the circuit does not contain any independent current or voltage sources. (c) Explain why a voltage follower implemented with an op-amp is a useful circuit component. (d) Explain how the nature of KCL allows the connection to a single op-amp input of multiple voltage signals, each through a resistor, to perform a summing function.