Assertion (A): When a resistance wire is stretched so as to make its length double, its resistance becomes 4 times of its original value.
Reason (R ): Resistance of a conductor is directly proportional to its length and inversely proportional to its cross section area.

Options

A. Both (A) and (R ) are true and (R ) is correct explanation of the assertion
B. Both (A ) and ( R) are true but (R ) is not the correct explanation of the assertion.
C. (A ) is true but (R ) is false
D. (A) is false but (R ) is true

Explanation

When a resistance wire is stretched so as to make its length double it means that `l.=2l` How ever, as volume of wire must remain unchanged, hence cross section area of wire is
reduced to one half of its original value i.e., `A.=A/2` SO the new resistance
`R.=(rho l.)/A.=(rho(2l))/((A//2))=4(rho l)/A=4R`.