For comparing the e.m.f.'s of two cells with a potentiometer, a standard cell is used to develop a potential gradient along the wires. Which of the following possibilities would make the experiment unsuccessful?

• A

  The e.m.f. of the standard cell is larger than the E e.m.f.'s of the two cells.

• B

  The diameter of the wires is the same and uniform throughout.

• C

  The number of wires is ten.

• D

  The e.m.f. of the standard cell is smaller than the e.m.f.'s of the two cells.