A compound A has prominent infrared absorptions at 1050, 1786, and 1852 cm-1 and shows a single absorption in the proton NMR spectrum at ? 3.00. When heated gently with methanol, compound B, C5H8O4, is obtained. Compound B has IR absorptions at 2500-3000 (broad), 1730, and 1701 cm-1, and its proton NMR spectrum in D2O consists of resonances at ? 2.7 (complex splitting) and ? 3.7 (a singlet) in the intensity ratio 4:3. Give the structures A and B, omitting stereochemistry.