Where we want to "cover" n elements with the fewest set possible. Let OPT be the optimal number of sets picked to cover all elements. What is the tightest lower bound of the size of the first set...

Where we want to "cover" n elements with the fewest set possible. Let OPT be the optimal number of sets picked to cover all elements. What is the tightest lower bound of the size of the first set picked by the greedy algorithm?