Difference of two sets

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

1. A and B are disjoint sets
2. A is contained in B
3. A and B are both Null Sets
4. B - A is a Null Set.

Q.

For 2 sets A and B, which of the following is/are true always?

1. A - B = {x : x ∈ A and x ∉ B}

2. The sets A - B, A ∩ B and B - A are mutually disjoint sets.

3. A - B = B - A

4. A ∪ B = {x : x ∈ B and x ∉ A}

Q. Let A and B be two sets in the same universal set. Then A - B is a null set if

1. A and B are disjoint sets
2. A is contained in B
3. B is contained in A
4. A and B are both Null Sets