(2019·新疆适应性测试)Superstorm Sandy slammed into the eastern United States three years ago.The winddriven water caused a lot of flooding，especially in the region's subway system.Now two teens have invented a device that could help prevent future subway floods.
In New York City alone，Superstorm Sandy caused an estimated $19 billion in damage.Much of the floodwaters entered the subway system through its ventilation grates(通风栅)．Those metal grills usually lie at ground level.When the system is shut down ahead of a flood，engineers try to cover those grates with plywood(胶合板) and sandbags.But the flooding often washes away those measures，which allows water to flow into the tunnels.Raising those grates even a tiny bit above ground level would cost a lot of money.Other ideas have included building tall chimneys around the holes.That would raise the grates and cost a lot of money，too.
Kyle and Raymond came up with a different，less expensive idea.They designed a metal grate with a builtin，adjustable cover.When trains are running，one part of the grate slides open to bring in fresh air.But when the system is shut down，that part can be slid shut to completely block the grate.
Their grate is the same size as the grates used now.So，it would be simple，they say，to lift out the old grate and drop in a replacement.Their design includes tamperproof features，which can prevent operation mistakes.They make sure that people don't intentionally shut off airflow during normal conditions.For people with the proper key，it would only take a minute or two to close the grate ahead of a flood.
1．What's the second paragraph mainly about?
A．The steps to operate grates.
B．The measures to prevent flood.
C．The function of plywood and sandbags.
D．The disadvantage of present subway system.
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