The fluorescence of A is quenched by 10% in the presence of 10 mM of B. If the fluorescence lifetime of A in the absence of B is 5 ns, the rate constant (in M-1 s-1) for interaction between B and photo-excited A is
1
1.2 × 109
2
2.2 × 109
3
3.2 × 109
4
4.2 × 109