My discretization is not very good, but when solving this kind of problem, I will draw the Haas diagram of this set A first, and use it to solve it more intuitively.

In the upper and lower bound problem, there is such a property:

If it contains an upper bound, it must contain a minimum upper bound; In contrast, if there is a lower limit, there must be a maximum lower limit.

Then, for the first question, "2" should not exist for all elements in "b" "If all x∈B" in the definition exist x ≤ y ",that is, 2 has a partial ordering relation of divisors for 3 and 9, so it cannot be regarded as a lower bound.

For example, B = {2.3.4.6} in the first question, then 8.9 is not the upper bound of this B set.

The answer level is limited. Please correct me if there are any mistakes!