A circular coil of wire consisting of 100 times of radius $8.0\;cm$ carries a current of $0.40\;A$ What is the magnitude of the magnetic field B at the center of the coil ?

Question

$\begin{array}{1 1} 3.1 \times 10^{-4}\;T \\ 3.1 \times 10^{-3}\;T \\ 3.1 \times 10^{4}\;T \\ 3.1 \times 10^{3}\;T\end{array} $