The equivalent resistance between A and B is

 

• A

  20 ohm if $V_A>V_B$

• B

  30 ohm if$V_A>V_B$

• C

  zero if ${\mathrm{V}}_{\mathrm{A}}>{\mathrm{V}}_{\mathrm{B}} \mathrm{and} 50 \mathrm{ohm} \mathrm{if} {\mathrm{V}}_{\mathrm{A}} <{\mathrm{V}}_{\mathrm{B}}$

• D

  zero if ${\mathrm{V}}_{\mathrm{A}}>{\mathrm{V}}_{\mathrm{B}}\mathrm{and} 50 \mathrm{ohm} \mathrm{if} {\mathrm{V}}_{\mathrm{A}}<{\mathrm{V}}_{\mathrm{B}}$