讲解下这道题---**Extraction Content:** **Question 1:** 如图所示的电路，开关 S 闭合，正常工作一段时间后，电流表示数突然减小但不为零。 用电压表检测，当电压表接 a、b 两点时，电压表有示数；接 b、c 两点时，电压表无示数，则故障是（ ） **Options:** A. 开关 S 接触不良 B. ac 导线断路 C. R₁ 断路 D. R₂ 断路 **Chart/Diagram Description:** * **Type:** Circuit diagram. * **Main Elements:** * **Power Source:** A battery is shown with positive and negative terminals. * **Ammeter:** An ammeter labeled 'A' is connected in series in the main circuit, on the positive side of the battery. * **Resistors:** Two resistors are shown in the upper branch. R₁ is placed between points 'a' and 'b'. R₂ is placed between points 'b' and 'c'. The resistors are represented by rectangular symbols. * **Switch:** A switch labeled 'S' is connected from point 'b' to the wire connecting point 'c' to the negative terminal of the battery. It appears to be a single-pole, single-throw switch shown in the closed position according to the question description. * **Points/Nodes:** Points 'a', 'b', and 'c' are labeled on the wires. Point 'a' is located after the ammeter and before R₁. Point 'b' is located between R₁ and R₂. Point 'c' is located after R₂ and is connected back to the negative terminal of the battery. * **Connecting Wires:** Straight lines represent connecting wires between components and points. The main circuit loop goes from the battery, through the ammeter, to 'a', through R₁, through 'b', through R₂, to 'c', and back to the battery. There is a separate connection via the switch S from point 'b' to the wire returning from 'c' to the battery's negative terminal.