$.$ A ring of radius $R_1$ is centered at the origin and carries an increasing current $)>(0,$ counterclockwise direction in the $xy$ plane as shown in the figure. A second, much smaller loop of radius $R_2$ is placed parallel to the firs loop with its center at $z=h$ What is the magnitude of the magnetic field created by the bottom loop at the center of the top loop?

Select one:

a$.,B_z=B_y=0,B_x=\frac{{}_0{R}_1^{2}t}{2(R_1^2+h^2{)}^{\frac{3}{2}}}$

b$.,B_x=B_y=0,B_z=\frac{{}_0{R}_1^{2}t}{2(R_1^2+h^2{)}^{\frac{3}{2}}}$

c$.,B_x=B_y=0,B_z=\frac{{}_0{R}_1^{2}t}{2(R_1^2+h^2{)}^{\frac{3}{2}}}.$

d$.,B_z=B_y=0,B_x=\frac{{}_0{R}_1^2}{(R_1^2+h^2{)}^{\frac{3}{2}}}$