A current carrying rectangular loop PQRS is made of uniform wire. The length $PR = QS =5\,cm$ and $PQ = RS =100\,cm$. If ammeter current reading changes from I to $2 I$, the ratio of magnetic forces per unit length on the wire $P Q$ due to wire RS in the two cases respectively $f_{ PQ }^{ I }: f_{ PQ }^{2 I }$ is :
A$1: 2$
B$1: 4$
C$1: 5$
D$1: 3$
JEE MAIN 2023, Medium
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B$1: 4$
b $F \propto I _1 I _2$
$F _{ I }: F _{2 I }=1: 4$
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