at least one mounting plate with a plurality of openings arranged in a row along a first direction;

a rotatable shaft located adjacent to the at least one mounting plate, an axis of the rotatable shaft being substantially parallel to the first direction;

a plurality of blocking plates pivotably attached at edges of the plurality of openings; and

a plurality of jacking members attached to the rotatable shaft, wherein each of the plurality of jacking members protrudes from the shaft along a second direction which is substantially perpendicular to the axis of the shaft; lengths along the second direction of the plurality of jacking members are different from each other; when the shaft is rotated to an initial position, the plurality of jacking members disengage away from the plurality of blocking plates, and the plurality of blocking plates are received in the plurality of openings; when the shaft is rotated to a working position, the plurality of blocking plates is rotated out of the plurality of openings to positions slanted relative to the at least one mounting plate by the plurality of jacking members, and slanting angles of the plurality of blocking plates relative to the at least one mounting plate are different from each other.