The lesson's rule is to choose a structure by which operations dominate the workload. Why is 'which structure is fastest?' the wrong question to ask in the first place?

A. Because no structure is fastest at everything; each concedes something.

The question is malformed rather than difficult: a hash table cannot produce the smallest element, a tree's lookup is logarithmic, an array's middle insertion is O(n). Every structure is a trade, and the workload — which operations, in what proportion, at what size — is what makes the question answerable. It is also why a composed structure is a legitimate answer rather than a hack.