单项选择题

With Airbus’s giant A380 airliner about in to take to the skies, you might think planes could not get much bigger — and you would be right. For a given design, it turns (21) , there comes a point where the wings become too heavy to generate (22) lift to carry their own weight. (23) a new way of designing and making materials could (24) that problem. Two engineers (25) University College London have devised an innovative way to customise and control the (26) of a material throughout its three-dimensional structure.
In the (27) of a wing, this would make possible a material that is dense, strong and load-bearing at one end, close to the fuselage, (28) the extremities could be made less dense, lighter and more (29) . It is like making bespoke materials, (30) you can customise the physical properties of every cubic millimetre of a structure.
The new technique combines existing technologies in a(n) (31) way. It starts by using finite-element-analysis software, of the type commonly used by engineers, (32) a virtual prototype of the object. The software models the stresses and strains that the object will need to (33) throughout its structure. Using this information it is then (34) to calculate the precise forces acting on millions of smaller subsections of the structure. (35) of these subsections is (36) treated as a separate object with its own set of forces acting on it — and each subsection (37) for a different microstructure to absorb those local forces.
Designing so many microstructures manually (38) be a huge task, so the researchers apply an optimisation program, called a genetic algorithm, (39) This uses a process of randomization and trial-and-error to search the vast number of possible microstructures to find the most (40) design for each subsection.