I ask you to sit down ___________ table, not to sit down ___________ the table.
A：at; in
B：at; at
C：on; to
D：in; in
读《开国大典》（节选）回答问题
综合全文看，***有怎样的品质？
P是四边形ABCD内一点，PA=PB=PC=PD，又AB=CD，试确定四边形ABCD的形状，并加以证明．
