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, 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 centre was built. Shopping centres, or rather malls, （12） as a collection of small new stores （13） crowded city centres. 14 by hundreds of free parking space, customers were drawn away from （14） areas to outlying malls. And the growing （16） of shopping centres led （17） to the building of bigger and better瞫tocked 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. 请在11处填上正确答案（）

A. when
B. while
C. since
D. the