Analysis: If p q is the sufficient condition of P and Q is the necessary condition of P, when we call closing switch A as condition P and light bulb B as conclusion Q, we can get the correct answer by combining simple electrical knowledge. For example, closing switch A or switch C( 1) in the figure can make the light bulb B light. Conversely, if the bulb B is to be lit, it is not necessary to close the switch A.. Therefore, closing the switch A is a sufficient but not necessary condition for the light bulb B to light. Topic (2) Turn on the switch A without turning on the switch C, and the light bulb B does not light up. Conversely, if the light bulb B is to be lit, the switch A must be closed, which means that closing the switch A is a necessary but not sufficient condition for the light bulb B to be lit. Topic (3) Turning on switch A can make light bulb B light, but light bulb B lights up. Switch a must be closed, so closing switch a is a necessary and sufficient condition for bulb b to light up. Topic (4) If switch A is turned on instead of switch C, light bulb B will not light up, and vice versa, it is not necessary to turn on switch A. Turning on switch C can show that turning on switch A is an insufficient and unnecessary condition for light bulb B to light up.