A large, flat circular coil with \(N_1\) turns and radius \(R\) lies in a plane. A much smaller, flat circular coil with \(N_2\) turns and radius \(r\) (where \(r \ll R\)) is placed at the center of the large coil and is coplanar with it. In terms of \(N_1\), \(N_2\), \(R\), \(r\), and fundamental constants, which of the following expressions represents the mutual inductance \(M\) of the two-coil system?