There are two long coaxial solenoids of same length l. The inner and outer coils have radii r1 and r2 and number of turns per unit length n1 and n2, respectively. The ratio of mutual inductance to the self-inductance of the inner coil is

• A. \(\frac{{{n_2}}}{{{n_1}}}\frac{{{r_1}}}{{{r_2}}}\)
• B. \(\frac{{{n_2}}}{{{n_1}}}\frac{{r_2^2}}{{r_1^2}}\)
• C. \(\frac{{{n_2}}}{{{n_1}}}\)
• D. \(\frac{{{n_1}}}{{{n_2}}}\)