Four squares of sides $x\: cm$ each are cut off from the four corners of a square metal sheet having side $10\: cm.$ The residual sheet is then folded into an open box which is then filled with a liquid costing Rs. $x^{2}$ per $cm^{3}.$ The value of $x$ for which the cost of filling the box completely with the liquid is maximized, is
- $100$
- $50$
- $30$
- $10$
