Question

Let A and B be two sets in the same universal set. Then A - B = A if

• A. A and B are disjoint sets
• B. A is contained in B
• C. A and B are both Null Sets
• D. B - A is a Null Set.

Answer 1

Answer: (A), (C)

The correct options are
A A and B are disjoint sets
C A and B are both Null Sets

A - B = A
$\Rightarrow A-(A\cap B)=A\Rightarrow A\cap B\mathrm{is}a\mathrm{null}\mathrm{set}.$
Therefore, A and B are disjoint sets.

If A and B are null sets, then A - B is also a null set which is equal to A.