For almost forty years the Empire State Building in New York City was the world's tallest skyscraper . 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 called rivets  . 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. In steel -frame building, the bottom walls  _   .
must be very thick must be rounded are usually red - hot do not need to be thick
Question: For almost forty years the Empire State Building in New York City was the world's tallest skyscraper . 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 called rivets  . 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. 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
Answer: