在矩形ABCD中，对角线AC与BD相交于点O，点E、F分别是AB，AO的中点，且AC=8。则EF的长度为（ ）---**Image Description:** * **Type:** Geometric figure. * **Main Elements:** * A rectangle ABCD is shown, with vertices labeled A, B, C, and D in a counter-clockwise manner starting from the top-left corner. * The diagonals AC and BD are drawn, intersecting at point O. * Point E is located on the side AB. * A line segment EF is drawn from point E to a point F on the diagonal AC. * Labels: Points are labeled A, B, C, D, E, F, O. Lines/segments are AB, BC, CD, DA, AC, BD, EF. * **Relative Position and Direction:** * ABCD is a rectangle. * AC and BD are the diagonals of the rectangle. * O is the intersection point of diagonals AC and BD. * E is a point on side AB. * F is a point on diagonal AC. * Segment EF connects E on AB to F on AC.