Wheatstone's bridge is a very simple but powerfull concept which has made many calculations easy. If the assumption is made that current does not flow through the galvanometer from B to C or C to B,
`V_(A)-V_(B)=V_(A)-V_(D)` `V_(B)-V_(D)=V_(C)-V_(D)`

The current in R1, has to flow through BD and that in `R_(3)` through CD. This idea has been extended to many symmetrical circuits.
For Wheatstone's bridge,

Options

A. it is necessary that `R_(1) = R_(2)` and `R_(3) = R_(4)`
B. it is necessary that the current in `R_(1)` should be equal to current in `R_(3)`
C. it is not necessary that the current in `R_(1)` should be equal to the current in `R_(3)`.
D. the current in `R_(2)` should be the same that in `R_(4)`

Answer

C