A couple of clarifications on the homework.
By optimal, I mean one that uses fewer sets from C than any other cover. In class we saw an example with fire hydrants on a graph, but the graph was rather large. In this problem you look for a smaller solution. (A smaller example may do better in capturing the essence of how the algorithm is suboptimal.)
A spice can be divided into fractional parts. If you the thief wanted to pick up half of one spice, a third of another, and all of another, that would be fine (assuming that this is the most valuable combination of spices).