[单项选择]城市道路的路基填方范围内的管涵顶面回填作业，采用压路机碾压时填土厚度至少为（）。
A. 300
B. 500
C. 800
D. 1000

{{B}}Questions 47 to 51 are based on the following passage.{{/B}}
Tulips were introduced into Holland before the 17th century but it did not take long for the flowers to gain popularity among the upper classes. Flowers of such beauty and rarity soon be came symbols of power and prestige and the rich tried their utmost to lay their hands on some to display in their gardens.
By 1634, the whole country was so fascinated by tulips that all other activities almost came to a stop. People were trading in tulips and even buying and selling unsprouted flowers. It was similar to the futures market today, where traders are buying and selling crude oil or cotton which they will never see. As the tulip trades increased, regular markets were set up on the Stock Exchange of Amsterdam and other towns. That happened in the year 1636 when the craze was reaching its peak.
The bubble finally burst in 1637. For some unknown reason