\textbf{185.} A long straight wire carries a current of $20\,\text{A}$. The magnetic field at a distance of $10\,\text{cm}$ from this wire is how many gauss?
$$\left(\mu_\circ = 4\pi \times 10^{-7}\,\frac{\text{T.m}}{\text{A}}\right)$$
\begin{itemize}
\item[(1)] $4\times10^{-3}$
\item[(2)] $4\times10^{-1}$
\item[(3)] $4\pi\times10^{-5}$
\item[(4)] $4\pi\times10^{-2}$
\end{itemize}