An element, $d l=d x$ \hat{ i } (where $d x=1 \mathrm{~cm}$ ) is placed at the origin and carries a large current $i=10 \mathrm{~A}$. What is the magnetic field on the $y$-axis at a distance of $0.5 \mathrm{~m} ?$
(a) $2 \times 10^{-8} \hat{\mathrm{k}} \mathrm{T}$
(b) $4 \times 10^{-8} \hat{\mathrm{k}} \mathrm{T}$
(c) $-2 \times 10^{-8} \hat{\mathrm{k}} \mathrm{T}$
(d) $-4 \times 10^{-8} \hat{\mathrm{k}} \mathrm{T}$