For almost forty years the Empire State Building in New York City was the world's tallest skyscraper（摩天大楼）. The building reaches a height of 1, 250 feet. It has 102 floors. The use of steel frames made its height possible. Steel walls can rise height without needing to be thick. Steel frame construction is strong. All plates are fixed firmly together by metal bolts（螺栓）called rivets （铆钉）. When set in place, rivets are usuall red- hot. One end is already rounded. A riveter uses a special hammer to shape the other end. Then the rivet cools. It holds the plates firmly. High winds can move this strong building. A wind of a hundred miles an hour has caused it to move almost one and a half inches.
　The writer says that the Empire State Building ________.

A．hasn’t been finished

B．is in New York City

C．is the tallest skyscraper in the world now

D．had fewer floors than the New World Trade Center

　　　In steel -frame building, the bottom walls________ .

A．must be very thick

B．must be rounded

C．are usually red - hot

D．do not need to be thick

　　　Rivets are described as being ________.

A．picked up by tools when red -hot

B．hit by tools when red - hot

C．usually caught in a hole

D．usually put in place when red - hot