The following sentences form a paragraph. The first and the last sentences of the paragraph are given. The rest of the sentences are numbered as P, Q, R and S. These four parts are not given in their proper order. Read the sentences and choose the alternative that arranges them in correct order.

1. Willis Tower is a skyscraper in Chicago, Illinois.

P. Sears, Roebuck & Company paid for it to be built in August 1970.

Q. The building was finished in 1973.

R. It was the tallest building in the world from 1973 to 1996.

S. From the ground to the roof it is 442 meters (1,450 feet) tall.

6. Until July 16, 2009, the building was called Sears Tower.