kevsim,
After you have a go at sib's question, here is one more. D and L are often seen in reference to biomolecules and have something to do with stereochemistry. D-glucose is the biologically important isomer, but L-glucose is not. Likewise, L-arginine is biologically important, but D-arginine is rarely, if ever, seen in biological systems. The meanings of the D and L prefixes are often misunderstood by students, and so it is worth getting them right.