$\begin{array}{1 1} (a)\;B=\large\frac{\mu_0I}{4 \pi a}\bigg[\large\frac{\sin \phi_1}{\sin \phi_2}\bigg]\\ (b)\;B=\large\frac{\mu_0I} {4 \pi} a \\ (c)\;B=\large\frac{\mu_0I}{a}\bigg[\sin \phi_1+\sin \phi_2\bigg] \\ (d)\;B=\large\frac{\mu_0}{I}\bigg[\large\frac{\sin \phi_1}{\sin \phi_2}\bigg] \end{array} $
