[多选题]装置引入高压氮气时，其管线上的各级（    ）应正常投用。
A.排气阀
B.导淋阀
C.减压阀
D.压力表

In the two decades between 1910 and 1930, over ten percent of the Black population of the United States left the South, where the majority of the Black population had been located, and migrated to northern states, with the largest number moving, it is claimed, between 1916 and 1918. It has been frequently assumed, but not proved, that most of the migrants in what has come to be called the Great Migration came from rural areas and were motivated by two concurrent factors: the collapse of cotton industry following boll weevil infestation, which began in 1898, and increased demand in the North for labor following the cessation of European immigration caused by the outbreak of the First World War in 1914. This assumption has led to the conclusion that the migrants’ subsequent lack of economic mobility in the North is tied to rural background, a background that implies unfamiliarity with urban living and a lack of industrial skills.
But the question of who actually left
A. The effect of migration on the regional economies of the United States following the First World War
B. The reasons for the subsequent economic difficulties of those who participated in the Great Migration
C. The transition from an urban existence for those who migrated in the Great Migration
D. The transformation of the agricultural South following the boll weevil infestation