Correct choice is (c) The module is not perfectly rigid
Easy explanation: In ASOM, by suitable adjustment of the roof block the opening height can be changed easily after dismantling the masonry around it. But the roof block cannot be readjusted without breaking the masonry around it and thus, the outlet discharge cannot be easily tempered by the cultivators. The adjustment in dimensions has to be done at a slight cost of re-doing the masonry and hence, the module is perfectly rigid.