Shopping habits in the United States have changed greatly in the last quarter of the 20th century. 　1　in the 1900s most American towns and cities had a Main Street. Main Street was always in the heart of a town. This street was 　2　on both sides with many 　3　businesses. Here, shoppers walked into stores to look at all sorts of merchandise: clothing, furniture, hardware, and groceries. 　4　, some shops offered 　5　. These shops included drugstores, restaurants, shoe repair stores, and barber or hairdressing shops. 　6　in the 1950s, a change began to 　7　. Too many automobiles had crowded into Main Street 　8　too few parking places were 　9　shoppers. Because the streets were crowded, merchants began to look with interest at the open spaces 　10　the city limits. Open space is what their car-driving customers needed. And open space is what they got 　11　the first shopping center was built. Shopping centers, or rather malls, 　12　as a collection of small new stores 　13　crowded city centers. 　14　by hundreds of free parking space, customers were drawn away from 　15　areas to outlying malls. And the growing 　16　of shopping centers led 　17　to the building of bigger and better stocked stores. 　18　the late 1970s, many shopping malls had almost developed into small cities themselves. In addition to providing the 　19　of one-stop shopping, malls were transformed into landscaped parks, 　20　benches, fountains, and outdoor entertainment.
______

A.when 

B.while  

C.since

D.then