帮我解答题目---**Extracted Content:** **Question Stem:** 6. 如图, 某小组设计了灯泡亮度可调的电路, a、b、c 为固定的三个触点, 理想变压器原、副线圈匝数比为 k, 灯泡 L 和三个电阻的阻值均恒为 R, 交变电源输出电压的有效值恒为 U。开关 S 与不同触点相连, 下列说法正确的是 (□□) **Diagram Description:** * **Type:** Circuit diagram. * **Main Elements:** AC power source, switch S, three resistors (labeled R), ideal transformer, lamp L. * **Connections:** * An AC power source is connected to a switch S. * The switch S can connect to three fixed contacts: a, b, and c. * Three resistors, each with resistance R, are connected in series. Let's call them R1, R2, and R3 from top to bottom. * Contact a is connected to the top end of R1. * Contact b is connected to the junction between R1 and R2. * Contact c is connected to the junction between R2 and R3. * The transformer's primary winding is connected across the series combination of R1, R2, and R3 (from the top end of R1 to the bottom end of R3). * The secondary winding of the transformer is connected to the lamp L. * **Transformer:** Ideal transformer. The primary winding is on the left, and the secondary winding is on the right. The ratio of secondary turns to primary turns is k. This means the turns ratio from primary to secondary is 1:k. * **Labels:** S, a, b, c, R, L, U (implied source voltage). **Options:** A. S 与 a 相连, 灯泡的电功率最大 B. S 与 a 相连, 灯泡两端的电压为 $\frac{kU}{k^2+3}$ C. S 与 b 相连, 流过灯泡的电流为 $\frac{U}{(k^2+2)R}$ D. S 与 c 相连, 灯泡的电功率为 $\frac{U^2}{(k^2+1)R}$