Directions : Read the passage and choose the best alternative to answer each question. (Item 1-7)
    Aircraft need a lot of power to fly slowly and a lot to fly fast, but they can afford to cruise along at intermediate speeds. But is this true for flying animals? After all, it is surprising that many flying animals, from bees to birds, are able to fly at all, let alone fly in the same way as fixed-wing aircraft do.
    In the latest issue of the science magazine Nature, Kenneth Dial and colleagues have, for the first time, succeeded in measuring the power output of flying birds, at all speeds, by taking direct readings from fight muscles. The importance of this study lies in the extreme practical difficulty of measuring the power output of a flying animal.
    All studies concentrate on respirometry – that is, measuring the rate at which an animal consumes oxygen as it flies. This requires fitting some kind of mask to the animal. This poses obvious problems for birds, emphasized for hummingbirds (because of their smallness) and practically impossible for insects (which in any case breathe through pores all over the body.
    A masked bird will not necessarily behave in the same way as an unmasked bird. The solution is to bypass this indirect strategy and measure the power output of birds directly. Dial and colleagues did this by fitting tiny strain-meters to the upper wing-bones of anaesthetized magpies, wired up to a plug on the back of the bird. This allowed the researchers to measure the forces generated by the bird’s wing as it flew, independently of its oxygen consumption.
    The results were something of a surprise. Although the birds used a lot of power when hovering at slow speeds, their power output at high speeds was hard more than that expended while cruising.

Aircraft use the least power when they are……