knapsack problem

Given ?? items, with ??th item worth ???? dollars and weighing ???? kilograms. A thief wants to take as valuable a load as possible, but he can carry at most ?? kilograms in his knapsack. Suppose this thief is allowed to take any fraction of items. Prove that this fractional knapsack problem.

a) the greedy-choice property.

b) and the optimal substructure property