Statements : All needles are threads. All threads are boxes. All trees are boxes.

Conclusions :
I. No needle is tree.
II. Some trees are threads.
III. Some boxes are needles.
IV. Some trees are needles.

All needles are threads. All threads are boxes.

Since both the premises are universal and affirmative, the conclusion must be universal affirmative (A-type) and should not contain the middle term. So, it follows that 'All needles are boxes'. III is the converse of this conclusion and so it holds.

All threads are boxes. All trees are boxes.

Since the middle term 'boxes' is not distributed even once in the premises, no definite conclusion follows.

All needles are boxes. All trees are boxes.

Again, since the middle term 'boxes' is not distributed even once in the premises, no definite conclusion can be drawn. However, I and IV involve the extreme terms of these two statements and form a complementary pair. Thus, either I or IV follows.