A freshly prepared solution of pure alpha-D-glucopyranose has specific rotation +112 degrees, which falls on standing to a constant +52.7 degrees. The best explanation is

1. Ainterconversion of the alpha and beta anomers through the open-chain aldehyde form until an equilibrium mixture forms
2. Bconversion of D-glucose into L-glucose
3. Cconversion of glucose into fructose
4. Dslow air oxidation of glucose