【简答题】
A、Z<sub>2sub>Z<sub>3sub>/(Z<sub>2sub>+Z<sub>3sub>) B、Z<sub>1sub>+Z<sub>2sub>Z<sub>3sub>/(Z<sub>2sub>+Z<sub>3sub>) C、Z<sub>1sub>+Z<sub>2sub>+Z<sub>3sub> D、Z<sub>1sub>+(Z<sub>2sub>+Z3)/(Z<sub>2sub>Z<sub>3sub>)