Two identical strings, A and B, have nearly the same tension. When they both vibrate in their fundamental resonant modes, there is a beat frequency of 3 Hz. When string B is tightened slightly, to increase the tension, the beat frequency becomes 6 Hz. This means: 

A. that before tightening A had a higher frequency than B, but after tightening, B has a higher frequency than A 

B. that before tightening B had a higher frequency than A, but after tightening, A has a higher frequency than B 

C. that before and after tightening A has a higher frequency than B 

D. that before and after tightening B has a higher frequency than A 

E. none of the above