In a common base configuration the emitter current changes by 5mA when emitter voltage is changed by 200 mV at a fixed collector to base voltage. The input resistance is

• A

  $40\mathrm{\Omega }$

• B

  $1000\mathrm{\Omega }$

• C

  $2.5\mathrm{\Omega }$

• D

  $4\mathrm{\Omega }$