Let and be subsets of the -plane, and consider the following statements.
: For all real numbers and , is in if and only if is in .
: There exists a point that lies in both and .
Which of the following statements is true?
A is necessary and sufficient for .
B is necessary but not sufficient for .
C is sufficient but not necessary for .
D is not necessary and not sufficient for .
E is sufficient but not necessary for .
F and are logically equivalent.
G implies whenever is non-empty.
HBoth and are always true.